ftp_sendquote: use PPSENDF, not FTPSENDF

The last remaining code piece that still used FTPSENDF now uses PPSENDF.
In the problematic case, a PREQUOTE series was done on a re-used
connection when Curl_pp_init() hadn't been called so it had messed up
pointers. The init call is done properly from Curl_pp_sendf() so this
change fixes this particular crash.

Bug: http://curl.haxx.se/mail/lib-2013-03/0319.html
Reported by: Sam Deane