krb5: Fix reachable assert when importing krb5 names

If a name token contains trailing garbage, error out from
krb5_gss_import_name() instead of crashing the process with an
assertion failure.

Commit message details obtained from upstream commit.
Obtained from:	upstream commit 07818f1fd
Reported by:	Aisle Research (Ze Sheng, Dmitrijs Trizna,
		Luigino Camastra, Guido Vranken) to krb5-bugs
MFC after:	3 days

crypto/krb5/src/lib/gssapi/krb5/import_name.c

@@ -297,7 +297,8 @@ import_name(OM_uint32 *minor_status, gss_buffer_t input_name_buffer,
                     goto fail_name;
                 cp += length;
             }
-            assert(cp == end);
+            if (cp != end)
+		goto fail_name;
         } else {
             status = GSS_S_BAD_NAMETYPE;
             goto cleanup;