Fix an out-of-bounds read in `png_image_read_composite`

Add a defensive bounds check before calling PNG_sRGB_FROM_LINEAR to
prevent reading up to 506 entries (1012 bytes) past `png_sRGB_base[]`.

For palette images with gamma, `png_init_read_transformations`
clears PNG_COMPOSE after compositing on the palette, but it leaves
PNG_FLAG_OPTIMIZE_ALPHA set. The simplified API then calls
`png_image_read_composite` with sRGB data (not linear premultiplied),
causing the index to reach 1017. (The maximum valid index is 511.)

NOTE:
This is a defensive fix that addresses the security issue (out-of-bounds
read) but *NOT* the correctness issue (wrong output). When the clamp
triggers, the affected pixels are clamped to white instead of the
correct composited color. Valid PNG images may render incorrectly with
the simplified API.

TODO:
We already know the root cause is a flag synchronization error.
For palette images with gamma, `png_init_read_transformations`
clears PNG_COMPOSE but leaves PNG_FLAG_OPTIMIZE_ALPHA set, causing
`png_image_read_composite` to misinterpret sRGB data as linear
premultiplied. However, we have yet to implement an architectural fix
that requires coordinating the simplified API with the transformation
pipeline.

Reported-by: flyfish101 <flyfish101@users.noreply.github.com>

pngread.c

diff --git a/pngread.c b/pngread.c
index 79917da..ab62edd 100644
--- a/pngread.c
+++ b/pngread.c
@@ -3273,9 +3273,14 @@
                         component += (255-alpha)*png_sRGB_table[outrow[c]];
 
                         /* So 'component' is scaled by 255*65535 and is
-                         * therefore appropriate for the sRGB to linear
-                         * conversion table.
+                         * therefore appropriate for the sRGB-to-linear
+                         * conversion table.  Clamp to the valid range
+                         * as a defensive measure against an internal
+                         * libpng bug where the data is sRGB rather than
+                         * linear premultiplied.
                          */
+                        if (component > 255*65535)
+                           component = 255*65535;
                         component = PNG_sRGB_FROM_LINEAR(component);
                      }