Fix CVE-2017-5981:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-5981

Patch copied from Debian.
Index: zziplib-0.13.62/zzip/fseeko.c
===================================================================
--- zziplib-0.13.62.orig/zzip/fseeko.c
+++ zziplib-0.13.62/zzip/fseeko.c
@@ -311,7 +311,8 @@ zzip_entry_findfirst(FILE * disk)
             } else
                 continue;
 
-            assert(0 <= root && root < mapsize);
+	    if (root < 0 || root >= mapsize)
+	        goto error;
             if (fseeko(disk, root, SEEK_SET) == -1)
                 goto error;
             if (fread(disk_(entry), 1, sizeof(*disk_(entry)), disk)
