Fix gss_set_sec_context_option() context creation

If gss_set_sec_context_option() creates a union context object, it
must set the loopback pointer.

[ghudson@mit.edu: extracted this from a larger commit and wrote commit
message]

(cherry picked from commit 93b6d2a83849e157d7bec4f83fd94dbef10d6f05)

https://github.com/krb5/krb5/commit/7b9fd4d4e736490bacbf00e6bb94fab3fe8914e8
Author: Luke Howard <lukeh@padl.com>
Committer: Greg Hudson <ghudson@mit.edu>
Commit: 7b9fd4d4e736490bacbf00e6bb94fab3fe8914e8
Branch: krb5-1.16
 src/lib/gssapi/mechglue/g_set_context_option.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)