runtime: fix global buffer reset in StopTrace

We reset global buffer only if its pos != 0.
We ought to do it always, but queue it only if pos != 0.
This is a latent bug. Currently it does not fire because
whenever we create a global buffer, we increment pos.

Change-Id: I01e28ae88ce9a5412497c524391b8b7cb443ffd9
Reviewed-on: https://go-review.googlesource.com/25574
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/trace.go

@@ -262,11 +262,13 @@ func StopTrace() {
 			p.tracebuf = 0
 		}
 	}
-	if trace.buf != 0 && trace.buf.ptr().pos != 0 {
+	if trace.buf != 0 {
 		buf := trace.buf
 		trace.buf = 0
+		if buf.ptr().pos != 0 {
 			traceFullQueue(buf)
 		}
+	}
 
 	for {
 		trace.ticksEnd = cputicks()