Pointers used before validation

After analyzing and filtering out the false positives from a static analyzer's report, there are three places left where a pointer is being used/dereferenced before being checked against NULL:

1. lib/isc/netmgr/netmgr.c - uvreq in isc__nm_async_readcb()
2. lib/isc/netmgr/tcpdns.c - ievent->sock in isc__nm_async_tcpdnssend()
3. lib/isccfg/parser.c - obj in cfg_print_spacelist()