krb5 commit: Fix potential memory leak in recent commit

https://github.com/krb5/krb5/commit/654f257b8843e3d85e368d8816511d99a8bab987
commit 654f257b8843e3d85e368d8816511d99a8bab987
Author: Greg Hudson <[email protected]>
Date:   Sun Mar 26 01:32:57 2023 -0400

    Fix potential memory leak in recent commit
    
    In get_negTokenInit(), reject a zero-length mechTypes field before
    copying it into *der_mechSet, to prevent allocating a zero-length GSS
    buffer.

 src/lib/gssapi/spnego/spnego_mech.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/lib/gssapi/spnego/spnego_mech.c b/src/lib/gssapi/spnego/spnego_mech.c
index bdd75868a..5923c880b 100644
--- a/src/lib/gssapi/spnego/spnego_mech.c
+++ b/src/lib/gssapi/spnego/spnego_mech.c
@@ -3437,8 +3437,9 @@ get_negTokenInit(OM_uint32 *minor_status,
 	if (!k5_der_get_value(&seq, SEQUENCE, &seq))
 		return GSS_S_DEFECTIVE_TOKEN;
 
-	/* Get the contents of the mechTypes field. */
-	if (!k5_der_get_value(&seq, CONTEXT, &field))
+	/* Get the contents of the mechTypes field.  Reject an empty field here
+	 * since we musn't allocate a zero-length buffer in the next step. */
+	if (!k5_der_get_value(&seq, CONTEXT, &field) || field.len == 0)
 		return GSS_S_DEFECTIVE_TOKEN;
 
 	/* Store a copy of the contents for MIC computation. */