Fix #77270: imagecolormatch Out Of Bounds Write on Heap

At least some of the image reading functions may return images which
use color indexes greater than or equal to im->colorsTotal.  We cater
to this by always using a buffer size which is sufficient for
`gdMaxColors` in `gdImageColorMatch()`.

ext/gd/libgd/gd_color.c

@@ -33,8 +33,8 @@ int gdImageColorMatch (gdImagePtr im1, gdImagePtr im2)
 		return -4; /* At least 1 color must be allocated */
 	}
 
-	buf = (unsigned long *)safe_emalloc(sizeof(unsigned long), 5 * im2->colorsTotal, 0);
-	memset( buf, 0, sizeof(unsigned long) * 5 * im2->colorsTotal );
+	buf = (unsigned long *)safe_emalloc(sizeof(unsigned long), 5 * gdMaxColors, 0);
+	memset( buf, 0, sizeof(unsigned long) * 5 * gdMaxColors );
 
 	for (x=0; x<im1->sx; x++) {
 		for( y=0; y<im1->sy; y++ ) {

ext/gd/tests/bug77270.phpt

@@ -0,0 +1,18 @@
+--TEST--
+Bug #77270 (imagecolormatch Out Of Bounds Write on Heap)
+--SKIPIF--
+<?php
+if (!extension_loaded('gd')) die('skip gd extension not available');
+if (!GD_BUNDLED && version_compare(GD_VERSION, '2.2.5', '<=')) die('skip upstream bugfix has not been released');
+?>
+--FILE--
+<?php
+$img1 = imagecreatetruecolor(0xfff, 0xfff);
+$img2 = imagecreate(0xfff, 0xfff);
+imagecolorallocate($img2, 0, 0, 0);
+imagesetpixel($img2, 0, 0, 255);
+imagecolormatch($img1, $img2);
+?>
+===DONE===
+--EXPECT--
+===DONE===