runtime: sigpanic in scanframeworker

[tbg]

A crash with the following stack trace on go1.10.3 was reported anonymously from a CockroachDB server (8 CPUs):

*log.safeError: stopper.go:182: runtime.errorString: runtime error: invalid memory address or nil pointer dereference
  File "runtime/asm_amd64.s", line 573, in call32
  File "runtime/panic.go", line 502, in gopanic
  File "runtime/panic.go", line 63, in panicmem
  File "runtime/signal_unix.go", line 388, in sigpanic
  File "runtime/mgcmark.go", line 827, in scanframeworker

My best reading of this is that scanframeworker code tried to log and instead hit an NPE (not sure where though, frame shouldn't be nil as it's already dereferenced earlier in the code):

go/src/runtime/mgcmark.go

Line 827 in fe8a0d1

print("runtime: frame ", funcname(f), " untyped locals ", hex(frame.varp-size), "+", hex(size), "\n")

If this hadn't NPEd, the stack trace would've been something like #17499.

Unfortunately, due to the nature of the report, I can neither reproduce this nor do I have more information about the environment in which this happened.

[bcmills]

CC @aclements @RLH

[tbg]

I should add that this happened on linux-amd64.