This should fix the intermittent calling write barrier with mp.p == nil
failures on the nacl/386 builder.

Change-Id: I34aef5ca75ccd2939e6a6ad3f5dacec64903074e
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/7973Reviewed-by: Austin Clements <austin@google.com>