EAP-pwd peer: Fix asymmetric fragmentation behavior

EAP-pwd peer: Fix asymmetric fragmentation behavior

The L (Length) and M (More) flags needs to be cleared before deciding
whether the locally generated response requires fragmentation. This
fixes an issue where these flags from the server could have been invalid
for the following message. In some cases, this could have resulted in
triggering the wpabuf security check that would terminate the process
due to invalid buffer allocation.

Signed-off-by: Jouni Malinen <j@w1.fi>