I opened the door to grotty (virused ?) gifs too far,
this closes it a little.

Reference: /n/sources/patch/applied/gif-robustness
Date: Fri Mar 24 13:17:40 CET 2006
Signed-off-by: steve@quintile.net

--- /sys/src/cmd/jpg/readgif.c	Fri Mar 24 13:13:08 2006
+++ /sys/src/cmd/jpg/readgif.c	Fri Mar 24 13:13:05 2006
@@ -451,7 +451,7 @@
 				fprint(2, "ReadGIF: GIF invalid, code out of range, %x > %x\n", code, nentry);
 				code = nentry;
 			}
-			for(c=code; c>=0; c=tbl[c].prefix)
+			for(c=code; stacki>0 && c>=0; c=tbl[c].prefix)
 				stack[stacki--] = tbl[c].exten;
 
 			nb = (sizeof stack)-(stacki+1);