runtime: clear up lr comments

R=cshapiro
CC=golang-dev
https://golang.org/cl/7230052

src/pkg/runtime/traceback_arm.c

@@ -32,7 +32,7 @@ runtime·gentraceback(byte *pc0, byte *sp, byte *lr0, G *gp, int32 skip, uintptr
 	waspanic = false;
 
 	// If the PC is goexit, the goroutine hasn't started yet.
-	if(pc == (uintptr)runtime·goexit) {
+	if(pc == (uintptr)runtime·goexit && gp->entry != 0) {
 		pc = (uintptr)gp->entry;
 		lr = (uintptr)runtime·goexit;
 	}

src/pkg/runtime/traceback_x86.c

@@ -40,19 +40,12 @@ runtime·gentraceback(byte *pc0, byte *sp, byte *lr0, G *gp, int32 skip, uintptr
 	waspanic = false;
 	
 	// If the PC is goexit, the goroutine hasn't started yet.
-	if(pc0 == gp->sched.pc && sp == (byte*)gp->sched.sp && pc0 == (byte*)runtime·goexit) {
+	if(pc0 == gp->sched.pc && sp == (byte*)gp->sched.sp && pc0 == (byte*)runtime·goexit && gp->entry != 0) {
 		fp = sp;
 		lr = pc;
 		pc = (uintptr)gp->entry;
 	}
 	
-	// If the PC is zero, it's likely a nil function call.
-	// Start in the caller's frame.
-	if(pc == 0) {
-		pc = lr;
-		lr = 0;
-	}
-
 	// If the PC is zero, it's likely a nil function call.
 	// Start in the caller's frame.
 	if(pc == 0) {