ldpd: fix processing of Label Withdraw messages

ldpd: fix processing of Label Withdraw messages

Whenever we receive a Label Withdraw message with an optional Label
TLV, we should check if this label matches the label previously
received from this neighbor for this FEC. If they don't match then we
shouldn't uninstall the previous label from the kernel. This fixes a
misinterpretation from the "Receive Label Withdraw" algorithm described
in the A.1.5 section of RFC 5036.

Also, simplify the check of pending withdraws in lde_check_release()
and lde_check_release_wcard().

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>