remove unnecessary assertion in dns_dispatch_connect()

When a thread calls dns_dispatch_connect() on an unconnected TCP socket
it sets `tcpstate` from `DNS_DISPATCHSTATE_NONE` to `_CONNECTING`.
Previously, it then INSISTed that there were no pending connections
before calling isc_nm_tcpdnsconnect().

If a second thread called dns_dispatch_connect() during that window
of time, it could add a pending connection to the list, and trigger
an assertion failure.

This commit removes the INSIST since the condition is actually
harmless.

(cherry picked from commit 25ddec8a0a)

CHANGES

@@ -1,3 +1,7 @@
+5927.	[bug]		A race was possible in dns_dispatch_connect()
+			that could trigger an assertion failure if two
+			threads called it near-simultaneously. [GL #3456]
+
 5926.	[func]		Handle transient TCP connect() EADDRINUSE failures
 			on FreeBSD (and possibly other BSDs) by trying three
 			times before giving up. [GL #3451]

lib/dns/dispatch.c

@@ -1812,7 +1812,6 @@ dns_dispatch_connect(dns_dispentry_t *resp) {
 		case DNS_DISPATCHSTATE_NONE:
 			/* First connection, continue with connecting */
 			LOCK(&disp->lock);
-			INSIST(ISC_LIST_EMPTY(disp->pending));
 			ISC_LIST_APPEND(disp->pending, resp, plink);
 			UNLOCK(&disp->lock);
 			dns_dispatch_attach(disp, &(dns_dispatch_t *){ NULL });