MINOR: check: Fix checks when using SRV records.

When started, a server may not yet have an associated protocol, so don't
bother trying to run the checks until it is there.
diff --git a/src/checks.c b/src/checks.c
index 7938b87..fc92a24 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -1557,7 +1557,7 @@
 	}
 
 	ret = SF_ERR_INTERNAL;
-	if (proto->connect)
+	if (proto && proto->connect)
 		ret = proto->connect(conn, check->type, quickack ? 2 : 0);
 	if (s->check.send_proxy && !(check->state & CHK_ST_AGENT)) {
 		conn->send_proxy_ofs = 1;