lib/ofp-util: Restore the check for minus sign in port number strings.

Commit 33ab38d9 (meta-flow: Simplify mf_from_ofp_port_string())
inadvertently removed a check for minus sign at the beginning of a
port number string introduced by commit 05dddba (meta-flow: Don't
allow negative port numbers).  This check is still needed, so put it
back, but to ofputil_port_from_string() this time.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>
Reviewed-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>