Initialize conn->Pfdebug to NULL when creating a connection

Failing to do this can cause a crash, and I suspect is what has happened
with a buildfarm member reporting mysterious failures.

This is an ancient bug, but I'm not backpatching since evidently nobody
cares about PQtrace in older releases.

Discussion: https://postgr.es/m/3333908.1617227066@sss.pgh.pa.us

src/interfaces/libpq/fe-connect.c

@@ -3952,6 +3952,7 @@ makeEmptyPGconn(void)
 	conn->verbosity = PQERRORS_DEFAULT;
 	conn->show_context = PQSHOW_CONTEXT_ERRORS;
 	conn->sock = PGINVALID_SOCKET;
+	conn->Pfdebug = NULL;
 
 	/*
 	 * We try to send at least 8K at a time, which is the usual size of pipe