Length check when parsing GSS token encapsulation

Length check when parsing GSS token encapsulation

gssint_get_mech_type_oid() is used by gss_accept_sec_context() to
determine the mechanism of the token.  Without length checking, it
might read a few bytes past the end of the input token buffer.  Add
length checking as well as test cases for truncated encapsulations.
Reported by Bar Katz.

(cherry picked from commit f949e990f930f48df1f108fe311c58ae3da18b24)

ticket: 8620
version_fixed: 1.14.7