cmd/8g: don't crash if Prog->u.branch is nil The code is copied from cmd/6g. Empirically, all branch targets are nil in this code so something is still wrong, but at least this stops 8g -S from crashing.
cmd/8g: don't crash if Prog->u.branch is nil
The code is copied from cmd/6g.
Empirically, all branch targets are nil in this code so
something is still wrong, but at least this stops 8g -S
from crashing.

Update issue 7178

LGTM=dave, iant
R=iant, dave
CC=golang-codereviews
https://codereview.appspot.com/58400043

firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,firebitsbr/golang-old