runtime/pprof: printStackRecord should check whether len(stk) == 0

[josharian]

In runtime/pprof/pprof.go, func printStackRecord, if len(stk) == 0, I think junk will be written to the profile. This could happen if a user calls Add on a custom profile with a skip value that is too large. See also #18788 (comment).

cc @matloob

[gopherbot]

CL https://golang.org/cl/36891 mentions this issue.