pubkey.c (meth__pss_encode): Fix the buffer allocation.

pubkey.c (meth__pss_encode): Fix the buffer allocation.

The code overestimated the buffer length by one bit, which would be fine
except that it then uses the buffer estimate to validate the requested
salt length, which permits an overly large salt when NBITS == 1 (mod 8).
The underlying logic doesn't get this wrong, and returns a null pointer
as a rejection, which causes a later segfault.

Fix and add tests.