Incorrect SNMP responses can cause spine to crash

[TheWitness]

Describe the bug

Spine crashes in some cases where the snmp agent returns incomplete data.

0x00005555555613e7 in snmp_get_base (current_host=current_host@entry=0x7fffd40048f0, snmp_oid=snmp_oid@entry=0x7fffd400a240 ".1.3.6.1.2.1.1.3.0", should_fail=should_fail@entry=true) at snmp.c:442

This is due to the response->variables being NULL. This causes a segmentation fault attempting to copy a null string.

Expected behavior

More reliable spine