BUG/MINOR: sock: Unclosed fd in case of connection allocation failure

If allocating a connection object failed right after a successful accept
on a listener, the new file descriptor was not properly closed.

This fixes GitHub issue #905.
It can be backported to 2.3.

(cherry picked from commit 25dd0ad1239627aa87d9ce25d268546037a97197)
Signed-off-by: Willy Tarreau <w@1wt.eu>
diff --git a/src/sock.c b/src/sock.c
index 9eff7f1..3ebcb14 100644
--- a/src/sock.c
+++ b/src/sock.c
@@ -161,6 +161,9 @@
 
  fail_conn:
 	sockaddr_free(&addr);
+	/* The accept call already succeeded by the time we try to allocate the connection,
+	 * we need to close it in case of failure. */
+	close(cfd);
  fail_addr:
 	ret = CO_AC_PAUSE;
 	goto done;