test: check portable error message on issue46234.go

issue46234.go expects an error output "segmentation violation",
which is UNIX-specific. Check for "nil pointer dereference"
instead, which is emitted by the Go runtime and should work on all
platforms.

Should fix Windows builders.

Change-Id: I3f5a66a687d43cae5eaf6a9e942b877e5a248900
Reviewed-on: https://go-review.googlesource.com/c/go/+/321072


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>