gif: Fix potential infinite loop

Author: jserv

src/image-gif.c

@@ -465,7 +465,8 @@ static void read_ext(twin_gif_t *gif)
 {
     uint8_t label;
 
-    read(gif->fd, &label, 1);
+    if (read(gif->fd, &label, 1) < 1)
+        return;
     switch (label) {
     case 0x01:
         read_plain_text_ext(gif);
@@ -494,11 +495,11 @@ static int gif_get_frame(twin_gif_t *gif)
     while (sep != ',') {
         if (sep == ';')
             return 0;
-        if (sep == '!')
-            read_ext(gif);
-        else
+        if (sep != '!')
+            return -1;
+        read_ext(gif);
+        if (read(gif->fd, &sep, 1) < 1)
             return -1;
-        read(gif->fd, &sep, 1);
     }
     if (read_image(gif) == -1)
         return -1;