Do not assert when debugging SNMP requests with long OIDs (#2150)

Do not assert when debugging SNMP requests with long OIDs (#2150)

    FATAL: assertion failed: MemBuf.cc: "new_cap > (size_t) capacity"

Also fixes repeated same-buffer snmpDebugOid() calls that used to
accumulate stale content in snmpTreeNext():

    snmpTreeNext: Current : .1.3.6.1.4.1.3495
    snmpTreeNext: Next : .1.3.6.1.4.1.3495.1.3.6.1.4.1.3495.1.1.1.0

Current snmpDebugOid() callers have been checked for compatibility with
this fix. Fixing snmpDebugOid() API is out of this surgical fix scope.