runtime: remove useless assignment in test code

Change-Id: I5fecdf52e9e3035ea8feb5768985ed5200dbd6af
Reviewed-on: https://go-review.googlesource.com/32752
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <martisch@uos.de>

src/runtime/crash_test.go

@@ -141,7 +141,6 @@ func buildTestProg(t *testing.T, binary string, flags ...string) (string, error)
 	cmd.Dir = "testdata/" + binary
 	out, err := testEnv(cmd).CombinedOutput()
 	if err != nil {
-		exe = ""
 		target.err = fmt.Errorf("building %s %v: %v\n%s", binary, flags, err, out)
 		testprog.target[name] = target
 		return "", target.err