imap-send: minimum leakfix

EVen with the minimum "no-op" invocation t1517 makes, "git imap-send"
leaks an empty strbuf it used to read a 0-byte string into.

There are a few other topics cooking in 'next' that plugs many
other leaks in this program, so let's minimally fix this one, barely
enough to make CI pass, leaving the rest for the other topic.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

imap-send.c

@@ -1543,6 +1543,7 @@ int cmd_main(int argc, const char **argv)
 	}
 
 	if (all_msgs.len == 0) {
+		strbuf_release(&all_msgs);
 		fprintf(stderr, "nothing to send\n");
 		return 1;
 	}