Fix read overruns in SPNEGO parsing

Fix read overruns in SPNEGO parsing

Fix three read overruns discovered by the GitHub Security Lab team
(GHSL-2023-016, GHSL-2023-017, and GHSL-2023-018) using OSS-Fuzz.

In get_mech_set(), error out if gss_add_oid_set_member() fails rather
than continue the loop and increment i past the current bound of
returned_mechSet.  In g_verify_neg_token_init(), check for zero-byte
sequences before reading tag bytes, and reduce cur_size by one to
account for the tag byte when calling gssint_get_der_length().

(cherry picked from commit 47c2a12830dbd7fb8e13c239ddc0ac74129a91f6)

ticket: 9085
version_fixed: 1.20.2