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

…sForP

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>

src/runtime/trace2status.go

@@ -88,7 +88,7 @@ func (w traceWriter) writeProcStatusForP(pp *p, inSTW bool) traceWriter {
 		// but it still owns the P (it's not in _Psyscall yet). The goroutine entering
 		// _Gsyscall is the tracer's signal that the P its bound to is also in a syscall,
 		// so we need to emit a status that matches. See #64318.
-		if w.mp.p.ptr() == pp && readgstatus(w.mp.curg)&^_Gscan == _Gsyscall {
+		if w.mp.p.ptr() == pp && w.mp.curg != nil && readgstatus(w.mp.curg)&^_Gscan == _Gsyscall {
 			status = traceProcSyscall
 		}
 	case _Psyscall: