[libpng16] Quieted Coverity issues in pngfix.c, png-fix-itxt.c, pngvalid.c,

pngstest.c, and pngimage.c. Most seem harmless, but png-fix-itxt
would only work with iTXt chunks with length 255 or less.

Fixed cexcept.h in which GCC 5 reported that one of the auto
variables in the Try macro needs to be volatile to prevent value
being lost over the setjmp, and fixed g++ build breaks (John Bowler).

contrib/libtests/pngvalid.c

@@ -4100,7 +4100,7 @@ make_error(png_store* volatile psIn, png_byte PNG_CONST colour_type,
 
    Try
    {
-      png_structp pp;
+      volatile png_structp pp;
       png_infop pi;
 
       pp = set_store_for_write(ps, &pi, name);