runtime: initialize crashFD to -1

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>