[libpng16] Added "/* SAFE */" comments in pngrutil.c and pngrtran.c

where warnings may be erroneously issued by code-checking applications.
Revised manual about changes in iTXt chunk handling made in libpng-1.6.0.

pngrutil.c

@@ -3694,7 +3694,7 @@ png_do_read_interlace(png_row_infop row_info, png_bytep row, int pass,
 
             for (i = 0; i < row_info->width; i++)
             {
-               png_byte v[8];
+               png_byte v[8]; /* SAFE; pixel_bytes does not exceed 64 */
                int j;
 
                memcpy(v, sp, pixel_bytes);