pygensec: Don't modify Python bytes objects

pygensec: Don't modify Python bytes objects

gensec_update() and gensec_unwrap() can both modify their input buffers
(for example, during the inplace RRC operation on GSSAPI tokens).
However, buffers obtained from Python bytes objects must not be modified
in any way. Create a copy of the input buffer so the original isn't
modified.

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
BUG: https://bugzilla.samba.org/show_bug.cgi?id=14817
(cherry picked from commit 6818d204897d0b7946dcfbedf79cd53fb9b3f159)