PKI Tidy deletes all certificates when safety_buffer is higher than 9223372036

[smedefind]

Describe the bug
When you pass tidy a safety_buffer that is higher than int64 when converted to nanoseconds, the tidy job will delete every certificate in the cert store.

To Reproduce

# /opt/vault write pki/issue/role common_name=username ttl=31557600

# /opt/vault list pki/certs
Keys
----
68-21-c2-42-47-5d-f9-c3-7a-73-fc-f3-c0-54-3e-ec-98-4d-a6-a8

# /opt/vault read -format json pki/cert/68-21-c2-42-47-5d-f9-c3-7a-73-fc-f3-c0-54-3e-ec-98-4d-a6-a8 | jq -r .data.certificate | openssl x509 -noout -text | grep Not\ After\ :
            Not After : May 26 00:59:11 2019 GMT

# /opt/vault write /pki/tidy tidy_cert_store=true safety_buffer='9223372036'
Success! Data written to: pki/tidy

# /opt/vault list pki/certs
Keys
----
68-21-c2-42-47-5d-f9-c3-7a-73-fc-f3-c0-54-3e-ec-98-4d-a6-a8

# /opt/vault write /pki/tidy tidy_cert_store=true safety_buffer='9223372037'
Success! Data written to: pki/tidy

# /opt/vault list pki/certs
No value found at pki/certs/

Expected behavior
Tidy should never delete unexpired certificates.

Environment:

• Vault Server Version (retrieve with vault status): 0.10.0
• Vault CLI Version (retrieve with vault version): Vault v0.10.0 ('5dd7f25f5c4b541f2da62d70075b6f82771a650d')
• Server Operating System/Architecture: LX Zone on Triton/Joyent