show bug in logging: erratic redirection to DLGLOBAL for invalid categories

Add a check to logging_test.c to show a bug: when a logging category value that
is out-of-bounds is passed to the logging system, the internal map_subsys()
function should remap that to DLGLOBAL. But in fact DLGLOBAL is -1 and the
function fails to map this to a proper positive array index, directly returning
-1 instead. This results in a negative array index and undefined behavior. A
sanitize build should catch this.

The bug is confirmed by the fact that logging_test.err stays the same
(hopefully) although a logging output should appear from this patch. The test
could as well segfault or anything else, it's a bit of a gamble.

This bug will be fixed along with the expectation in a subsequent patch.

Note: osmo_log_info->num_cat + 0 is also out-of-bounds, but there is a separate
bug there, so leaving this for another patch.

Change-Id: I161b6550fa204a872bad1abefee1a6155393fafd

tests/logging/logging_test.c

@@ -66,6 +66,8 @@ const struct log_info log_info = {
 	.filter_fn = test_filter,
 };
 
+extern struct log_info *osmo_log_info;
+
 int main(int argc, char **argv)
 {
 	struct log_target *stderr_target;
@@ -108,5 +110,11 @@ int main(int argc, char **argv)
 	select_output = 1;
 	DEBUGP(DRLL, "You should see this\n");
 	OSMO_ASSERT(filter_called == 5); /* called twice on output */
+
+	/* Make sure out-of-bounds category maps to DLGLOBAL */
+	log_parse_category_mask(stderr_target, "DLGLOBAL,1");
+	DEBUGP(osmo_log_info->num_cat + 1, "You should see this on DLGLOBAL (a)\n");
+	DEBUGP(osmo_log_info->num_cat + 100, "You should see this on DLGLOBAL (b)\n");
+
 	return 0;
 }