runtime: initialize crashFD to -1

crashFD defaults to the zero value of (surprise!) zero. Zero is a valid
FD, so on the first call to SetCrashOutput we actually close FD 0 since
it is a "valid" FD.

Initialize crashFD to -1, the sentinel for "no FD".

Change-Id: I3b108c60603f2b83b867cbe079f035c159b6a6ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/560776
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/runtime/proc.go

@@ -775,6 +775,7 @@ func schedinit() {
 	}
 
 	sched.maxmcount = 10000
+	crashFD.Store(^uintptr(0))
 
 	// The world starts stopped.
 	worldStopped()

src/runtime/runtime.go

@@ -244,6 +244,8 @@ func writeErrData(data *byte, n int32) {
 // set by debug.SetCrashOutput (see #42888). If it is a valid fd (not
 // all ones), writeErr and related functions write to it in addition
 // to standard error.
+//
+// Initialized to -1 in schedinit.
 var crashFD atomic.Uintptr
 
 //go:linkname setCrashFD