runtime: check if mp.curg is nil before readgstatus in writeProcStatusForP

runtime: check if mp.curg is nil before readgstatus in writeProcStatusForP

CL 546025 failed to check if mp.curg is nil, causing all sorts of
failures. It can very well be nil in this context.

For #64318.
Fixes #64507.

Change-Id: I4a95c3fa16d5e1dee8041394c4bdb9c6ba04f032
Reviewed-on: https://go-review.googlesource.com/c/go/+/546636
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>