test: add limit configuration tests

nocompile-limits.dfa: turns off all limits including run-time limits

nolimits.dfa: makes the compile time limits unlimited while leaving on
    the run-time limits.

Fixes compiler warnings exposed by these tests. These are just warnings,
there were no bugs other than a failure to handle systems with a 16-bit
at the appropriate time which would result in a later failure on malloc.

png.c: png_icc_check_length: in-line code was still used in place of
    png_chunk_max when checking the current chunk allocation limit.  The
    in-line code did not handle PNG_MAXSEG_64K and, anyway, issued
    compiler warnings in the 'nocompile-limits' case.  Changed to use
    png_malloc_max.

pngrutil.c: eliminated an erroneous 'truncation' warning with GCC-14 by
    using a safe cast.

pngtest.c: failed to check for PNG_USER_LIMITS_SUPPORTED around API
    calls which don't exist without PNG_USER_LIMITS.

Signed-off-by: John Bowler <jbowler@acm.org>

contrib/conftest/nocompile-limits.dfa

@@ -0,0 +1,21 @@
+# nolimits.dfa
+#  Build time configuration of libpng
+#
+# Author: John Bowler
+# Copyright: (c) John Bowler, 2025
+#
+# Usage rights:
+#  To the extent possible under law, the author has waived all copyright and
+#  related or neighboring rights to this work.  This work is published from:
+#  United States.
+#
+# Build libpng without any limits and without run-time settable limits.  Turning
+# USER_LIMITS off reduces libpng code size by allowing compile-time elimination
+# of some checking code.
+#
+option USER_LIMITS off
+
+@# define PNG_USER_WIDTH_MAX PNG_UINT_31_MAX
+@# define PNG_USER_HEIGHT_MAX PNG_UINT_31_MAX
+@# define PNG_USER_CHUNK_CACHE_MAX 0
+@# define PNG_USER_CHUNK_MALLOC_MAX 0

contrib/conftest/nolimits.dfa

@@ -0,0 +1,19 @@
+# nolimits.dfa
+#  Build time configuration of libpng
+#
+# Author: John Bowler
+# Copyright: (c) John Bowler, 2025
+#
+# Usage rights:
+#  To the extent possible under law, the author has waived all copyright and
+#  related or neighboring rights to this work.  This work is published from:
+#  United States.
+#
+# Build libpng without any limits.  With these settigs run-time limits are still
+# possible.
+#
+@# define PNG_USER_WIDTH_MAX PNG_UINT_31_MAX
+@# define PNG_USER_HEIGHT_MAX PNG_UINT_31_MAX
+@# define PNG_USER_CHUNK_CACHE_MAX 0
+@# define PNG_USER_CHUNK_MALLOC_MAX 0
+

png.c

@@ -1600,21 +1600,9 @@ png_icc_check_length(png_const_structrp png_ptr, png_const_charp name,
     * the caller supplies the profile buffer so libpng doesn't allocate it.  See
     * the call to icc_check_length below (the write case).
     */
-#  ifdef PNG_SET_USER_LIMITS_SUPPORTED
-      else if (png_ptr->user_chunk_malloc_max > 0 &&
-               png_ptr->user_chunk_malloc_max < profile_length)
-         return png_icc_profile_error(png_ptr, name, profile_length,
-             "exceeds application limits");
-#  elif PNG_USER_CHUNK_MALLOC_MAX > 0
-      else if (PNG_USER_CHUNK_MALLOC_MAX < profile_length)
-         return png_icc_profile_error(png_ptr, name, profile_length,
-             "exceeds libpng limits");
-#  else /* !SET_USER_LIMITS */
-      /* This will get compiled out on all 32-bit and better systems. */
-      else if (PNG_SIZE_MAX < profile_length)
-         return png_icc_profile_error(png_ptr, name, profile_length,
-             "exceeds system limits");
-#  endif /* !SET_USER_LIMITS */
+   if (profile_length > png_chunk_max(png_ptr))
+      return png_icc_profile_error(png_ptr, name, profile_length,
+            "profile too long");
 
    return 1;
 }

pngrutil.c

@@ -4228,7 +4228,7 @@ png_read_IDAT_data(png_structrp png_ptr, png_bytep output,
          avail_in = png_ptr->IDAT_read_size;
 
          if (avail_in > png_chunk_max(png_ptr))
-            avail_in = png_chunk_max(png_ptr);
+            avail_in = (uInt)/*SAFE*/png_chunk_max(png_ptr);
 
          if (avail_in > png_ptr->idat_size)
             avail_in = (uInt)png_ptr->idat_size;

pngtest.c

@@ -2137,6 +2137,7 @@ main(int argc, char *argv[])
       fprintf(STDERR, " libpng FAILS test\n");
 
    dummy_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
+#ifdef PNG_USER_LIMITS_SUPPORTED
    fprintf(STDERR, " Default limits:\n");
    fprintf(STDERR, "  width_max  = %lu\n",
        (unsigned long) png_get_user_width_max(dummy_ptr));
@@ -2152,6 +2153,7 @@ main(int argc, char *argv[])
    else
       fprintf(STDERR, "  malloc_max = %lu\n",
           (unsigned long) png_get_chunk_malloc_max(dummy_ptr));
+#endif
    png_destroy_read_struct(&dummy_ptr, NULL, NULL);
 
    return (ierror != 0);