session: handle NULL return values of crypt()

session: handle NULL return values of crypt()

The crypt() function may return NULL with errno ENOSYS when an attempt
was made to crypt the plaintext password using a salt requesting an
unsupported cipher.

Avoid triggering segmentation faults in the subsequent strcmp() operation
by checking for a non-NULL hash value.

Fixes: FS#2291
Signed-off-by: Jo-Philipp Wich <jo@mein.io>