Move an sftp_state check inside mutex to silence Coverity.

There's still outstanding issues in Coverity around sftp_state and
sftp_channel that need some deep thought, but this one is trivial.

src/syncterm/ssh.c

@@ -605,14 +605,16 @@ add_public_key(void *vpriv)
 		}
 		sftp_channel = new_sftp_channel;
 		sftp_state = sftpc_begin(sftp_send, NULL);
-		pthread_mutex_unlock(&ssh_mutex);
-		pthread_mutex_unlock(&ssh_tx_mutex);
 		if (sftp_state == NULL) {
+			pthread_mutex_unlock(&ssh_mutex);
+			pthread_mutex_unlock(&ssh_tx_mutex);
 			close_sftp_channel(new_sftp_channel);
 			free(priv);
 			pubkey_thread_running = false;
 			return;
 		}
+		pthread_mutex_unlock(&ssh_mutex);
+		pthread_mutex_unlock(&ssh_tx_mutex);
 		if (sftpc_init(sftp_state)) {
 			sftp_filehandle_t f = NULL;
 			// TODO: Add permissions?