runtime: tricky replacements of g in traceback.go

Change-Id: I80e847451b697de049a01df051a73b5976b39d4a
Reviewed-on: https://go-review.googlesource.com/c/go/+/418589


Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/runtime/traceback.go

@@ -923,8 +923,8 @@ func gcallers(gp *g, skip int, pcbuf []uintptr) int {
 // showframe reports whether the frame with the given characteristics should
 // be printed during a traceback.
 func showframe(f funcInfo, gp *g, firstFrame bool, funcID, childID funcID) bool {
-	g := getg()
-	if g.m.throwing >= throwTypeRuntime && gp != nil && (gp == g.m.curg || gp == g.m.caughtsig.ptr()) {
+	mp := getg().m
+	if mp.throwing >= throwTypeRuntime && gp != nil && (gp == mp.curg || gp == mp.caughtsig.ptr()) {
 		return true
 	}
 	return showfuncinfo(f, firstFrame, funcID, childID)