easy: during upkeep, attach Curl_easy to connections in the cache

easy: during upkeep, attach Curl_easy to connections in the cache

During the protocol-specific parts of connection upkeep, some code
assumes that the data->conn pointer already is set correctly.  However,
there's currently no guarantee of that in the code.

This fix temporarily attaches each connection to the Curl_easy object
before performing the protocol-specific connection check on it, in a
similar manner to the connection checking in extract_if_dead().

Fixes #7386
Closes #7387
Reported-by: Josie Huddleston