avcodec/gif_parser: split correctly also bitstreams that do not have extension blocks

libavcodec/gif_parser.c

@@ -67,6 +67,17 @@ static int gif_find_frame_end(GIFParseContext *g, const uint8_t *buf,
                 g->state = GIF_EXTENSION;
                 g->found_start = pc->frame_start_found = 1;
             } else if (buf[index] == GIF_IMAGE_SEPARATOR) {
+                if (g->state != GIF_EXTENSION_BLOCK && g->found_start &&
+                    g->found_end && g->found_sig) {
+                    next = index;
+                    g->found_start = pc->frame_start_found = 1;
+                    g->found_end = 0;
+                    g->index = 0;
+                    g->gct_flag = 0;
+                    g->gct_size = 0;
+                    g->state = GIF_IMAGE;
+                    break;
+                }
                 g->state = GIF_IMAGE;
             } else if (buf[index] == GIF_TRAILER) {
                 g->state = 0;