cmd/gc: fix undefined behavior
UndefinedBehaviorSanitizer claims it is UB in C:
src/cmd/gc/racewalk.c:422:37: runtime error: member access within null pointer of type 'Node' (aka 'struct Node')
src/cmd/gc/racewalk.c:423:37: runtime error: member access within null pointer of type 'Node' (aka 'struct Node')

LGTM=rsc
R=dave, rsc
CC=golang-codereviews
https://codereview.appspot.com/125570043