Fix a possible dig/host crash in "NS search" mode

When getting a SERVFAIL reply from a query, 'host' tries to start the next query in the lookup's list (also true for 'dig +nofail'). However, when running with the '-C' switch (or +nssearch for 'dig'), all the queries in the lookup start from the beginning, so that logic brings to a crash because of the attempted start of the query which was already started.

Don't start the next query in the affected code path when in +nssearch mode.

Closes #4508 (closed)