kex1: Fix a possible null pointer dereference.

src/kex1.c

@@ -47,6 +47,9 @@ static ssh_string make_rsa1_string(ssh_string e, ssh_string n){
 
   buffer = ssh_buffer_new();
   rsa = ssh_string_from_char("ssh-rsa1");
+  if (rsa == NULL) {
+      goto error;
+  }
 
   if (buffer_add_ssh_string(buffer, rsa) < 0) {
     goto error;