Fix misuse of TextDatumGetCString().

Fix misuse of TextDatumGetCString().

"TextDatumGetCString(PG_GETARG_TEXT_P(x))" is formally wrong: a text*
is not a Datum.  Although this coding will accidentally fail to fail on
all known platforms, it risks leaking memory if a detoast step is needed,
unlike "TextDatumGetCString(PG_GETARG_DATUM(x))" which is what's used
elsewhere.  Make pg_get_object_address() fall in line with other uses.

Noted while reviewing two-arg current_setting() patch.