multi: fix off-by-one finit[] array size

introduced in c6aedf68. It needs to be CURLM_STATE_LAST big since it
must hande the range 0 .. CURLM_STATE_MSGSENT (18) and CURLM_STATE_LAST
is 19 right now.

Reported-by: Dan Fandrich
Bug: http://curl.haxx.se/mail/lib-2015-10/0069.html