std: it is important that func name is never an empty string

otherwise DTRACE_ANCHORED() returns false and that makes stack()
insert a bogus frame at the top.
For example:
dtrace -n 'test:dtrace_test::sdttest { stack(); }

This change is not really a solution, but just a work-around.
The real solution is to record the probe's call site and to use
that for resolving a function name.

PR:		195222
MFC after:	22 days

sys/cddl/dev/sdt/sdt.c

@@ -168,6 +168,8 @@ sdt_create_probe(struct sdt_probe *probe)
 	 * in the C compiler, so we have to respect const vs non-const.
 	 */
 	strlcpy(func, probe->func, sizeof(func));
+	if (func[0] == '\0')
+		strcpy(func, "none");
 
 	from = probe->name;
 	to = name;