Applied imagedeallocate() fix

ext/gd/gd.c

@@ -1867,6 +1867,13 @@ PHP_FUNCTION(imagecolordeallocate)
 
 	ZEND_FETCH_RESOURCE(im, gdImagePtr, IM, -1, "Image", le_gd);
 
+	#if HAVE_LIBGD20
+	/* We can return right away for a truecolor image as deallocating colours is meaningless here */
+	if (gdImageTrueColor(im)) {
+		RETURN_TRUE;
+	}
+	#endif
+
 	convert_to_long_ex(index);
 	col = Z_LVAL_PP(index);