SAE: Add forgotten commit element validation step for FFC groups

SAE: Add forgotten commit element validation step for FFC groups

The peer commit element needs to be validated to pass one of the steps
listed in IEEE 802.11, 11.3.5.4:
scalar-op(r, ELEMENT) = 1 modulo p

Similar step was present for ECC groups, but was missing for FFC groups.
This is needed to avoid dictionary attacks.

Thanks to Michael Roßberg and Sascha Grau for reporting this.

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