ftp wildcard: segfault due to init only in multi_perform

The proper FTP wildcard init is now more properly done in Curl_pretransfer()
and the corresponding cleanup in Curl_close().

The previous place of init/cleanup code made the internal pointer to be NULL
when this feature was used with the multi_socket() API, as it was made within
the curl_multi_perform() function.

Reported-by: Jonathan Cardoso Machado
Fixes #800