From: "Christoph M. Becker" <cmbecker69@gmx.de>
Date: Wed, 29 Nov 2017 19:37:38 +0100
Subject: Fix #420: Potential infinite loop in gdImageCreateFromGifCtx
origin: https://github.com/libgd/libgd/commit/a11f47475e6443b7f32d21f2271f28f417e2ac04
Bug-Debian-Security: https://security-tracker.debian.org/tracker/CVE-2018-5711
Bug-Debian: https://bugs.debian.org/887485
Bug: https://github.com/libgd/libgd/issues/420

Due to a signedness confusion in `GetCode_` a corrupt GIF file can
trigger an infinite loop.  Furthermore we make sure that a GIF without
any palette entries is treated as invalid *after* open palette entries
have been removed.

CVE-2018-5711

See also https://bugs.php.net/bug.php?id=75571.
---

--- a/src/gd_gif_in.c
+++ b/src/gd_gif_in.c
@@ -335,11 +335,6 @@ terminated:
 		return 0;
 	}
 
-	if(!im->colorsTotal) {
-		gdImageDestroy(im);
-		return 0;
-	}
-
 	/* Check for open colors at the end, so
 	 * we can reduce colorsTotal and ultimately
 	 * BitsPerPixel */
@@ -351,6 +346,11 @@ terminated:
 		}
 	}
 
+	if(!im->colorsTotal) {
+		gdImageDestroy(im);
+		return 0;
+	}
+
 	return im;
 }
 
@@ -447,7 +447,7 @@ static int
 GetCode_(gdIOCtx *fd, CODE_STATIC_DATA *scd, int code_size, int flag, int *ZeroDataBlockP)
 {
 	int i, j, ret;
-	unsigned char count;
+	int count;
 
 	if(flag) {
 		scd->curbit = 0;