sync: re-enable race even when panicking

Not doing this can cause user code running after this panic (e.g.:
defers) to produce non-existing races.

Change-Id: Ia6aec88aaeee3b9c17e7b8019d697ffa88dfb492
Reviewed-on: https://go-review.googlesource.com/c/go/+/713460
Commit-Queue: Nicolas Hillegeer <aktau@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Nicolas Hillegeer <aktau@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: aktau

src/sync/waitgroup.go

@@ -204,13 +204,14 @@ func (wg *WaitGroup) Wait() {
 				}
 			}
 			runtime_SemacquireWaitGroup(&wg.sema, synctestDurable)
-			if wg.state.Load() != 0 {
-				panic("sync: WaitGroup is reused before previous Wait has returned")
-			}
+			isReset := wg.state.Load() != 0
 			if race.Enabled {
 				race.Enable()
 				race.Acquire(unsafe.Pointer(wg))
 			}
+			if isReset {
+				panic("sync: WaitGroup is reused before previous Wait has returned")
+			}
 			return
 		}
 	}