If PQgetCancel() returns NULL we should also return false.

Noted by Andres Freund.

dbutils.c

@@ -443,8 +443,9 @@ CancelQuery(PGconn *conn, int timeout)
 
 	pgcancel = PQgetCancel(conn);
 
-	if (pgcancel != NULL)
-	{
+	if (pgcancel == NULL)
+		return false;
+
 	/*
 	 * PQcancel can only return 0 if socket()/connect()/send()
  	 * fails, in any of those cases we can assume something
@@ -458,7 +459,6 @@ CancelQuery(PGconn *conn, int timeout)
 	}
 
 	PQfreeCancel(pgcancel);
-	}
 
 	return true;
 }