The FTP connection caching needs a better interface -- connections are
closed through _fetch_close() which is the only one who knows the connection
REALLY was closed (since ref -> 0). However, FTP keeps its own local
cached_connection and checks if it is valid by comparing it to NULL. This
is bogus since it may have been freed elsewhere by _fetch_close().

This change checks if we are closing the cached_connection and the ref is 1
(soon to be 0). If so, set cached_connection to NULL so we don't
accidentally reuse it. The REAL fix should be to move connection caching
to the common.c level (_fetch_* functions) and NULL the cache(s) in
_fetch_close(). Then all layers could benefit from caching.