EAP-pwd: Enforce 1 < rand,mask < r and rand+mask mod r > 1

EAP-pwd: Enforce 1 < rand,mask < r and rand+mask mod r > 1

RFC 5931 has these conditions as MUST requirements, so better follow
them explicitly even if the rand,mask == 0 or rand+mask == 0 or 1 cases
are very unlikely to occur in practice while generating random values
locally.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>