mesh: return if peer commit scalar is NULL to avoid segfault

This checking is to ensure that we have received the commit message
from the peer mesh STA before doing sae_check_confirm.

Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@cozybit.com>
Merge pull request #3 from cozybit/yeow-fix-segfault

Avoid segfault if receiving confirm msg before commit msg