Handle cancel requests with PID 0 gracefully

If the client sent a query cancel request with backend PID 0, it
tripped an assertion. With assertions disabled, you got this in the
log instead:

    LOG:  invalid cancel request with PID 0
    LOG:  wrong key in cancel request for process 0

Query cancellations don't even require authentication, so we better
tolerate bogus requests. Fix by turning the assertion into a regular
runtime check.

Spotted while testing libpq behavior with a modified server that
didn't send BackendKeyData to the client.

Backpatch-through: 18

src/backend/storage/ipc/procsignal.c

@@ -728,7 +728,11 @@ procsignal_sigusr1_handler(SIGNAL_ARGS)
 void
 SendCancelRequest(int backendPID, const uint8 *cancel_key, int cancel_key_len)
 {
-	Assert(backendPID != 0);
+	if (backendPID == 0)
+	{
+		ereport(LOG, (errmsg("invalid cancel request with PID 0")));
+		return;
+	}
 
 	/*
 	 * See if we have a matching backend. Reading the pss_pid and