Curl_socket_check: enlarge poll struct array to 3

This function was introduced in commit 5527417a and as pointed out
by Gokhan Sengun, the array with poll structs must large enough to hold
3 sockets since that is what the function can accept. It could be noted
that he had this fixed in his patch as posted in
http://curl.haxx.se/mail/lib-2011-12/0179.html

Bug: http://curl.haxx.se/mail/lib-2011-12/0228.html
Reported by: Gokhan Sengun