runtime: check for nil g in msancall() on arm64

The current msanwrite() segfaults during libpreinit
when built with -msan on arm64. The cause is msancall()
in runtime/msan_arm64.s called by msanwrite() assumes
that it is always called with a valid g, leading to a
segfult.

This CL adds a check for nil g in msancall().

Fixes #34338

Change-Id: If4ad7e37556cd1d99346c1a7b4852651d1e4e4aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/196157Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/msan_arm64.s

@@ -47,9 +47,10 @@ TEXT	runtime·msanfree(SB), NOSPLIT, $0-16
 
 // Switches SP to g0 stack and calls (FARG). Arguments already set.
 TEXT	msancall<>(SB), NOSPLIT, $0-0
+	MOVD	RSP, R19                  // callee-saved
+	CBZ	g, g0stack                // no g, still on a system stack
 	MOVD	g_m(g), R10
 	MOVD	m_g0(R10), R11
-	MOVD	RSP, R19	// callee-saved
 	CMP	R11, g
 	BEQ	g0stack