fixup! fsmonitor: introduce core.useBuiltinFSMonitor to call the daemon via IPC

In FSMonitor v1, we made sure to only use a valid `since_token` when
querying the FSMonitor. This condition was accidentally lost in v2, and
caused segmentation faults uncovered by Scalar's Functional Tests.

I had tried to fix this in https://github.com/git-for-windows/pull/3241,
but the fix was incomplete, and I had to follow up with
https://github.com/git-for-windows/pull/3258. However, it turns out that
both of them were actually only work-arounds; I should have dug deeper
to figure out _why_ the `since_token` was no longer guaranteed not to be
`NULL`, and I finally did.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

fsmonitor.c

@@ -290,8 +290,9 @@ void refresh_fsmonitor(struct index_state *istate)
 	trace_printf_key(&trace_fsmonitor, "refresh fsmonitor");
 
 	if (r->settings.use_builtin_fsmonitor > 0) {
-		query_success = !fsmonitor_ipc__send_query(
-			istate->fsmonitor_last_update, &query_result);
+		query_success = istate->fsmonitor_last_update &&
+			!fsmonitor_ipc__send_query(istate->fsmonitor_last_update,
+						   &query_result);
 		if (query_success) {
 			/*
 			 * The response contains a series of nul terminated