Fix GH-17772: imagepalettetotruecolor segfault on invalid truecolor pixel.

Author: devnexen

ext/gd/libgd/gd.c

@@ -3108,7 +3108,7 @@ int gdImagePaletteToTrueColor(gdImagePtr src)
 		const unsigned int sy = gdImageSY(src);
 		const unsigned int sx = gdImageSX(src);
 
-		src->tpixels = (int **) gdMalloc(sizeof(int *) * sy);
+		src->tpixels = (int **) gdCalloc(sizeof(int *), sy);
 		if (src->tpixels == NULL) {
 			return 0;
 		}