file-gif-load: Fail early in several other places if GetDataBlocks() fails (Bug #737375)

plug-ins/common/file-gif-load.c

@@ -492,6 +492,11 @@ load_image (const gchar  *filename,
                                 GifScreen.Height);
         }
 
+      if (image_ID < 0)
+        {
+          break;
+        }
+
       if (comment_parasite != NULL)
         {
           if (! thumbnail)
@@ -656,7 +661,7 @@ GetDataBlock (FILE   *fd,
       return -1;
     }
 
-  ZeroDataBlock = count == 0;
+  ZeroDataBlock = (count == 0);
 
   if ((count != 0) && (! ReadOK (fd, buf, count)))
     {
@@ -698,8 +703,15 @@ GetCode (FILE     *fd,
       buf[0] = buf[last_byte - 2];
       buf[1] = buf[last_byte - 1];
 
-      if ((count = GetDataBlock (fd, &buf[2])) <= 0)
-        done = TRUE;
+      count = GetDataBlock (fd, &buf[2]);
+      if (count < 0)
+        {
+          return -1;
+        }
+      else if (count == 0)
+        {
+          done = TRUE;
+        }
 
       last_byte = 2 + count;
       curbit = (curbit - lastbit) + 16;
@@ -777,6 +789,11 @@ LZWReadByte (FILE *fd,
         }
       while (firstcode == clear_code);
 
+      if (firstcode < 0)
+        {
+          return -1;
+        }
+
       return firstcode & 255;
     }
 
@@ -804,6 +821,11 @@ LZWReadByte (FILE *fd,
           sp            = stack;
           firstcode     = oldcode = GetCode (fd, code_size, FALSE);
 
+          if (firstcode < 0)
+            {
+              return -1;
+            }
+
           return firstcode & 255;
         }
       else if (code == end_code || code > max_code)
@@ -865,6 +887,11 @@ LZWReadByte (FILE *fd,
         return (*--sp) & 255;
     }
 
+  if (code < 0)
+    {
+      return -1;
+    }
+
   return code & 255;
 }
 
@@ -1172,6 +1199,11 @@ ReadImage (FILE        *fd,
         break;
     }
 
+  if (v < 0)
+    {
+      return -1;
+    }
+
  fini:
   if (LZWReadByte (fd, FALSE, c) >= 0)
     g_print ("GIF: too much input data, ignoring extra...\n");