kex.c: make sure mlist is not set to NULL

... if the currently unsupported LANG methods are called.
Reported by Coverity CID 89834.

src/kex.c

@@ -1866,7 +1866,7 @@ libssh2_session_method_pref(LIBSSH2_SESSION * session, int method_type,
     }
     memcpy(s, prefs, prefs_len + 1);
 
-    while (s && *s) {
+    while (s && *s && mlist) {
         char *p = strchr(s, ',');
         int method_len = p ? (p - s) : (int) strlen(s);