managesieve: Fix missing-command check

Mirrors imap fix in Dovecot: 42149f48624b82fdf9631c256497580154c2e412

It's impossible for the command's pointer to be NULL at this point.
Previously, the command_find() would have returned NULL, but this check
presumably short-circuits that search in the trivial case, so has some
real use.

Problem now found by GCC 7.

src/managesieve/managesieve-client.c

@@ -616,7 +616,7 @@ static bool client_handle_input(struct client_command_context *cmd)
 		managesieve_refresh_proctitle();
 	}
 
-	if (cmd->name == '\0') {
+	if (cmd->name[0] == '\0') {
 		/* command not given - cmd_func is already NULL. */
 	} else {
 		/* find the command function */