s3-gse: Don't release the mech OID from gss_accept_security_context

s3-gse: Don't release the mech OID from gss_accept_security_context

This is constant data according to the man pages I find for this
fucntion, and causes a segfault to free() when linked to Heimdal.  I
am advised that while it is constant for gss_mech_krb5, it may not be
for other mechanisms, so an assert will ensure this is dealt with by
the programmer who extends this code in future.

Andrew Bartlett