Buffer overflow in the readextension function in gif2tiff.c
allows remote attackers to cause a denial of service via a crafted GIF file.

External References:
https://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2016-3186
https://bugzilla.redhat.com/show_bug.cgi?id=1319503

CVE: CVE-2016-3186
Upstream-Status: Backport (RedHat)
https://bugzilla.redhat.com/attachment.cgi?id=1144235&action=diff

Signed-off-by: Yi Zhao <yi.zhao@windirver.com>

--- tiff-4.0.6/tools/gif2tiff.c	2016-04-06 15:43:01.586048341 +0200
+++ tiff-4.0.6/tools/gif2tiff.c	2016-04-06 15:48:05.523207710 +0200
@@ -349,7 +349,7 @@
     int status = 1;
 
     (void) getc(infile);
-    while ((count = getc(infile)) && count <= 255)
+    while ((count = getc(infile)) && count >= 0 && count <= 255)
         if (fread(buf, 1, count, infile) != (size_t) count) {
             fprintf(stderr, "short read from file %s (%s)\n",
                     filename, strerror(errno));