runtime/debug: don't run a GC when setting SetGCPercent negative

If the user is calling SetGCPercent(-1), they intend to disable GC.
They probably don't intend to run one. If they do, they can call
runtime.GC themselves.

Change-Id: I40ef40dfc7e15193df9ff26159cd30e56b666f73
Reviewed-on: https://go-review.googlesource.com/34013
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/debug/garbage.go

@@ -90,7 +90,9 @@ func ReadGCStats(stats *GCStats) {
 // A negative percentage disables garbage collection.
 func SetGCPercent(percent int) int {
 	old := setGCPercent(int32(percent))
-	runtime.GC()
+	if percent >= 0 {
+		runtime.GC()
+	}
 	return int(old)
 }