curl: Make socket callback during cleanup into no-op

curl: Make socket callback during cleanup into no-op

Because curl_multi_cleanup may invoke callbacks, we effectively have
some circular references going on here. See discussion in

https://github.com/curl/curl/issues/14860

Basically what we do is the socket callback libcurl may invoke into a no-op when
we detect we're finalizing. The data structures are owned by this object and
not by the callbacks, and will be destroyed below. Note that
e.g. g_hash_table_unref() may itself invoke callbacks, which is where
some data is cleaned up.

Signed-off-by: Colin Walters <walters@verbum.org>