Check the GSS-API checksum exists before trying to use it [CVE-2010-1321]

This was introduced by checking the Kerberos 5 checksum as a
alternative to the 8003 checksum.

Thanks to MIT Kerberos and Shawn Emery for forwarding this issue

lib/gssapi/krb5/8003.c

@@ -182,11 +182,6 @@ _gsskrb5_verify_8003_checksum(
     int DlgOpt;
     static unsigned char zeros[16];
 
-    if (cksum == NULL) {
-	*minor_status = 0;
-	return GSS_S_BAD_BINDINGS;
-    }
-
     /* XXX should handle checksums > 24 bytes */
     if(cksum->cksumtype != CKSUMTYPE_GSSAPI || cksum->checksum.length < 24) {
 	*minor_status = 0;