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.

src/backend/catalog/objectaddress.c

@@ -1831,7 +1831,7 @@ textarray_to_strvaluelist(ArrayType *arr)
 Datum
 pg_get_object_address(PG_FUNCTION_ARGS)
 {
-	char	   *ttype = TextDatumGetCString(PG_GETARG_TEXT_P(0));
+	char	   *ttype = TextDatumGetCString(PG_GETARG_DATUM(0));
 	ArrayType  *namearr = PG_GETARG_ARRAYTYPE_P(1);
 	ArrayType  *argsarr = PG_GETARG_ARRAYTYPE_P(2);
 	int			itype;