runtime: have ReadMemStats do a nil check before switching stacks

This gives the user a better stack trace experience. No need to
expose them to runtime.systemstack and friends.

Fixes #61158

Change-Id: I4f423f82e54b062773067c0ae64622e37cb3948b
Reviewed-on: https://go-review.googlesource.com/c/go/+/507755
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>

src/runtime/mstats.go

@@ -347,6 +347,7 @@ func init() {
 // which is a snapshot as of the most recently completed garbage
 // collection cycle.
 func ReadMemStats(m *MemStats) {
+	_ = m.Alloc // nil check test before we switch stacks, see issue 61158
 	stopTheWorld(stwReadMemStats)
 
 	systemstack(func() {

src/runtime/traceback.go

@@ -1147,6 +1147,7 @@ func showfuncinfo(sf srcFunc, firstFrame bool, calleeID abi.FuncID) bool {
 
 // isExportedRuntime reports whether name is an exported runtime function.
 // It is only for runtime functions, so ASCII A-Z is fine.
+// TODO: this handles exported functions but not exported methods.
 func isExportedRuntime(name string) bool {
 	const n = len("runtime.")
 	return len(name) > n && name[:n] == "runtime." && 'A' <= name[n] && name[n] <= 'Z'