Curl_socket_check: fix return code for timeout

We found a problem with ftp transfer using libcurl (7.23 and 7.25)
inside an application which is receiving unix signals (SIGUSR1,
SIGUSR2...) almost continuously. (Linux 2.4, PowerPC, HAVE_POLL_FINE
defined).

Curl_socket_check() uses poll() to wait for the socket, and retries it
when a signal is received (EINTR). However, if a signal is received and
it also happens that the timeout has been reached, Curl_socket_check()
returns -1 instead of 0 (indicating an error instead of a timeout).

In our case, the result is an aborted connection even before the ftp
banner is received from the server, and a return value of
CURLE_OUT_OF_MEMORY from curl_easy_perform() (Curl_pp_multi_statemach(),
in pingpong.c, actually returns OOM if Curl_socket_check() fails :-)
Funny to debug on a system on which OOM is a possible cause).

Bug: http://curl.haxx.se/mail/lib-2012-07/0122.html