DateAndTime - Fallback when datetime = 0

[meni2029]

Re-insert the fallback when the datetime value is 0 (year and month = 0)

The plugin fails with mode hardware-health on Juniper SRX due to value of OID jnxOperatingRestartTime (1.3.6.1.4.1.2636.3.1.13.1.19) equal to "0-0-0,0:0:0.0,.0:0". It fails within the subfunction DateAndTime because epoch cannot be created with year and month = 0.

Plugin output:

# 'check_nwc_health' '--community' 'community' '--hostname' 'ipaddress ' '--mode' 'hardware-health' 
Month '-1' out of range 0..11 at check_nwc_health line 5804.

snmpwalk:

# snmpwalk -v2c -c community ipaddress 1.3.6.1.4.1.2636.3.1.13.1.19
JUNIPER-MIB::jnxOperatingRestartTime.1.1.0.0 = STRING: 2019-10-30,13:51:40.0,+0:0
JUNIPER-MIB::jnxOperatingRestartTime.2.1.0.0 = STRING: 2019-10-30,13:51:40.0,+0:0
JUNIPER-MIB::jnxOperatingRestartTime.7.1.0.0 = STRING: 2019-10-30,13:56:36.0,+0:0
JUNIPER-MIB::jnxOperatingRestartTime.8.1.1.0 = STRING: 2019-10-30,13:56:37.0,+0:0
JUNIPER-MIB::jnxOperatingRestartTime.9.1.0.0 = STRING: 2019-10-30,13:51:40.0,+0:0
JUNIPER-MIB::jnxOperatingRestartTime.9.1.1.0 = STRING: 0-0-0,0:0:0.0,.0:0

The piece of code added in this pull request was removed by commit dcdb758