kex1: Add missing NULL check in make_rsa1_string()

CID 1388445

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>
(cherry picked from commit c705fb6e3b)

src/kex1.c

@@ -50,6 +50,10 @@ static ssh_string make_rsa1_string(ssh_string e, ssh_string n){
   ssh_string ret = NULL;
 
   buffer = ssh_buffer_new();
+  if (buffer == NULL) {
+      goto error;
+  }
+
   rsa = ssh_string_from_char("ssh-rsa1");
   if (rsa == NULL) {
       goto error;