Teach the GIF library about read errors.

--- a/liblcdf/gifread.c
+++ b/liblcdf/gifread.c
@@ -57,7 +57,7 @@
   int is_record;
   int is_eoi;
   uint8_t (*byte_getter)(struct Gif_Reader *);
-  void (*block_getter)(uint8_t *, uint32_t, struct Gif_Reader *);
+  int (*block_getter)(uint8_t *, uint32_t, struct Gif_Reader *);
   uint32_t (*offseter)(struct Gif_Reader *);
   int (*eofer)(struct Gif_Reader *);
   
@@ -86,10 +86,10 @@
   return i == EOF ? 0 : (uint8_t)i;
 }
 
-static void
+int
 file_block_getter(uint8_t *p, uint32_t s, Gif_Reader *grr)
 {
-  fread(p, 1, s, grr->f);
+  return fread(p, 1, s, grr->f) == s;
 }
 
 static uint32_t
@@ -117,12 +117,16 @@
   return grr->w ? (grr->w--, *grr->v++) : 0;
 }
 
-static void
+static int
 record_block_getter(uint8_t *p, uint32_t s, Gif_Reader *grr)
 {
-  if (s > grr->w) s = grr->w;
+  int res;
+  res = (s <= grr->w);
+  if (!res)
+    s = grr->w;
   memcpy(p, grr->v, s);
   grr->w -= s, grr->v += s;
+  return res;
 }
 
 static uint32_t
@@ -215,7 +219,8 @@
     block_len = gifgetbyte(grr);
     GIF_DEBUG(("\nimage_block(%d)", block_len));
     if (block_len == 0) return 0;
-    gifgetblock(buffer + bit_length / 8, block_len, grr);
+    if (!gifgetblock(buffer + bit_length / 8, block_len, grr))
+      return 0;
     bit_length += block_len * 8;
   }
   
@@ -362,7 +367,8 @@
   i = gifgetbyte(grr);
   GIF_DEBUG(("\nafter_image(%d)\n", i));
   while (i > 0) {
-    gifgetblock(buffer, i, grr);
+    if (!gifgetblock(buffer, i, grr))
+      break;
     i = gifgetbyte(grr);
     GIF_DEBUG(("\nafter_image(%d)\n", i));
   }
@@ -477,7 +483,8 @@
 	if (!comp) return 0;
       }
       comp[comp_len] = i;
-      gifgetblock(comp + comp_len + 1, i, grr);
+      if (!gifgetblock(comp + comp_len + 1, i, grr))
+	break;
       comp_len += i + 1;
       i = gifgetbyte(grr);
     }
@@ -584,7 +591,8 @@
     uint8_t buffer[GIF_MAX_BLOCK];
     int i = gifgetbyte(grr);
     while (i > 0) {
-      gifgetblock(buffer, i, grr);
+      if (!gifgetblock(buffer, i, grr))
+	break;
       i = gifgetbyte(grr);
     }
   }
@@ -614,13 +622,15 @@
   
   if (len > 0) {
     gif_read_error(gfc, "odd graphic extension format");
-    gifgetblock(crap, len, grr);
+    if (!gifgetblock(crap, len, grr))
+      return;
   }
   
   len = gifgetbyte(grr);
   while (len > 0) {
     gif_read_error(gfc, "odd graphic extension format");
-    gifgetblock(crap, len, grr);
+    if (!gifgetblock(crap, len, grr))
+      break;
     len = gifgetbyte(grr);
   }
 }
@@ -638,7 +648,8 @@
   while (len > 0) {
     Gif_ReArray(data, char, total_len + len + 1);
     if (!data) return 0;
-    gifgetblock((uint8_t *)data, len, grr);
+    if (!gifgetblock((uint8_t *)data, len, grr))
+      break;
     
     total_len += len;
     data[total_len] = 0;
@@ -664,7 +675,8 @@
     if (data) Gif_ReArray(data, uint8_t, data_len + block_len + 1);
     else data = Gif_NewArray(uint8_t, block_len + 1);
     if (!data) goto done;
-    gifgetblock(data + data_len, block_len, grr);
+    if (!gifgetblock(data + data_len, block_len, grr))
+      break;
     data_len += block_len;
     block_len = gifgetbyte(grr);
   }
@@ -682,7 +694,8 @@
   if (!gfex) Gif_DeleteArray(data);
   while (block_len > 0) {
     uint8_t buffer[GIF_MAX_BLOCK];
-    gifgetblock(buffer, block_len, grr);
+    if (!gifgetblock(buffer, block_len, grr))
+      break;
     block_len = gifgetbyte(grr);
   }
   return gfex != 0;
@@ -695,7 +708,10 @@
   Gif_Stream *gfs = gfc->stream;
   uint8_t buffer[GIF_MAX_BLOCK + 1];
   uint8_t len = gifgetbyte(grr);
-  gifgetblock(buffer, len, grr);
+  if (!gifgetblock(buffer, len, grr)) {
+    gif_read_error(gfc, "bad application extension");
+    return 0;
+  }
   
   /* Read the Netscape loop extension. */
   if (len == 11 && memcmp(buffer, "NETSCAPE2.0", 11) == 0) {
@@ -710,7 +726,8 @@
       gif_read_error(gfc, "bad loop extension");
     
     while (len > 0) {
-      gifgetblock(buffer, len, grr);
+      if (!gifgetblock(buffer, len, grr))
+	break;
       len = gifgetbyte(grr);
     }
     return 1;