Check mech cred in gss_inquire_cred_by_mech()

If gss_inquire_cred_by_mech() is called with a mechanism and there is
no corresponding mechanism credential in the union cred, return
GSS_S_NO_CRED (as Heimdal does) instead of interrogating the mechanism
about the default credential.

(cherry picked from commit 8ea7e36661cfa6d8acb2b1af615870092a408cce)

https://github.com/krb5/krb5/commit/1b4558642544300b296b358e3de5fb683d2054c8
Author: Greg Hudson <ghudson@mit.edu>
Commit: 1b4558642544300b296b358e3de5fb683d2054c8
Branch: krb5-1.16
 src/lib/gssapi/mechglue/g_inq_cred.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)