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