asyn-thread: fix cppcheck warning

asyn-thread: fix cppcheck warning

- Check for NULL entry parameter before attempting to deref entry in
  Curl_resolver_is_resolved, like is already done in asyn-ares.

This is to silence cppcheck which does not seem to understand that
asyn-ares and asyn-thread have separate Curl_resolver_is_resolved
and those units are mutually exclusive. Prior to this change it warned
of a scenario where asyn-thread's Curl_resolver_is_resolved is called
with a NULL entry from asyn-ares, but that couldn't happen.

Reported-by: rl1987@users.noreply.github.com
Fixes https://github.com/curl/curl/issues/5326