kex: Avoid NULL pointer dereference (GHSL-2023-032)

Thanks Phil Turnbull from Github

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/kex.c

@@ -321,6 +321,10 @@ static int cmp_first_kex_algo(const char *client_str,
 
     int is_wrong = 1;
 
+    if (client_str == NULL || server_str == NULL) {
+        return is_wrong;
+    }
+
     colon = strchr(client_str, ',');
     if (colon == NULL) {
         client_kex_len = strlen(client_str);