runtime: handle nil gp in cpuprof

This can happen on Windows when recording profile samples for system threads.

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.
This CL is, however, not windows/arm64-specific.
It is cleanup meant to make the port (and future ports) easier.

Change-Id: I5a7ba32b1900a69f3b7acada9cb6cf8396d8a03f
Reviewed-on: https://go-review.googlesource.com/c/go/+/288797
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/cpuprof.go

@@ -103,7 +103,16 @@ func (p *cpuProfile) add(gp *g, stk []uintptr) {
 		// because otherwise its write barrier behavior may not
 		// be correct. See the long comment there before
 		// changing the argument here.
-		cpuprof.log.write(&gp.labels, nanotime(), hdr[:], stk)
+		//
+		// Note: it can happen on Windows, where we are calling
+		// p.add with a gp that is not the current g, that gp is nil,
+		// meaning we interrupted a system thread with no g.
+		// Avoid faulting in that case.
+		var tagPtr *unsafe.Pointer
+		if gp != nil {
+			tagPtr = &gp.labels
+		}
+		cpuprof.log.write(tagPtr, nanotime(), hdr[:], stk)
 	}
 
 	atomic.Store(&prof.signalLock, 0)