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 #7178

LGTM=dave, iant
R=iant, dave
CC=golang-codereviews
https://golang.org/cl/58400043

src/cmd/8g/list.c

@@ -107,7 +107,10 @@ Dconv(Fmt *fp)
 		break;
 
 	case D_BRANCH:
-		snprint(str, sizeof(str), "%d", a->u.branch->loc);
+		if(a->u.branch == nil)
+			snprint(str, sizeof(str), "<nil>");
+		else
+			snprint(str, sizeof(str), "%d", a->u.branch->loc);
 		break;
 
 	case D_EXTERN: