runtime: fix sigprof frame counting

If sigtramp and sigtrampgo are both on stack, n -= framesToDiscard
is executed twice, which should actually run only once.

Change-Id: Iec53059be4e4e010bd91ced93f03f1210436e694
Reviewed-on: https://go-review.googlesource.com/c/159238
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
1 file changed