Fix crash when KV store has a zero-length key.

[mgritter]

Unit test successfully demonstrates the crash:

--- FAIL: TestCoreMetrics_KvSecretGauge_BadPath (0.03s)
panic: runtime error: index out of range [-1] [recovered]
	panic: runtime error: index out of range [-1]

goroutine 9 [running]:
testing.tRunner.func1.1(0x2e2d400, 0xc0007a8680)
	/usr/local/go/src/testing/testing.go:1057 +0x30d
testing.tRunner.func1(0xc000643e00)
	/usr/local/go/src/testing/testing.go:1060 +0x41a
panic(0x2e2d400, 0xc0007a8680)
	/usr/local/go/src/runtime/panic.go:969 +0x175
github.com/hashicorp/vault/vault.(*Core).walkKvMountSecrets(0xc0001de600, 0x35b39e0, 0xc000703a10, 0xc0007039e0)
	/home/mgritter/vault/vault/core_metrics.go:297 +0xa94
github.com/hashicorp/vault/vault.(*Core).kvSecretGaugeCollector(0xc0001de600, 0x35b39e0, 0xc000703a10, 0xc0007038f0, 0x0, 0x0, 0x5, 0xc000798840)
	/home/mgritter/vault/vault/core_metrics.go:332 +0xd3
github.com/hashicorp/vault/vault.TestCoreMetrics_KvSecretGauge_BadPath(0xc000643e00)
	/home/mgritter/vault/vault/core_metrics_test.go:175 +0x478
testing.tRunner(0xc000643e00, 0x3190700)
	/usr/local/go/src/testing/testing.go:1108 +0xef
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:1159 +0x386