tee: fix use of possibly-freed pad in debug statement

tee: fix use of possibly-freed pad in debug statement

The gst_object_unref() in the block above may be dropping
the last ref to the pad and free the pad. Set pad pointer
to NULL here, so that we don't accidentally use a
possibly-freed pad pointer in the debug log statements
further below, and also use the tee element as log object
since that's more appropriate anyway.

Fixes valgrind warnings and crashes in tee test_stress
unit test when debug logging is enabled.