runtime/pprof: handle empty stack traces in Profile.Add

runtime/pprof: handle empty stack traces in Profile.Add

If the caller passes a large number to Profile.Add,
the list of pcs is empty, which results in junk
(a nil pc) being recorded. Check for that explicitly,
and replace such stack traces with a lostProfileEvent.

Fixes #18836.

Change-Id: I99c96aa67dd5525cd239ea96452e6e8fcb25ce02
Reviewed-on: https://go-review.googlesource.com/36891
Reviewed-by: Russ Cox <rsc@golang.org>