During a crash showing goroutine stacks of all threads
(with GOTRACEBACK=crash), it can be that f == nil.

Only happens on Solaris; not sure why.

Change-Id: Iee2c394a0cf19fa0a24f6befbc70776b9e42d25a
Reviewed-on: https://go-review.googlesource.com/17110Reviewed-by: Austin Clements <austin@google.com>