[PATCH] contrib/seg: Fix PG_GETARG_SEG_P definition

Hi hackers,

I just noticed that when contrib/seg was converted to V1 calling
convention (commit 389bb2818f4), the PG_GETARG_SEG_P() macro got defined
in terms of PG_GETARG_POINTER().  But it itself calls DatumGetPointer(),
so shouldn't it be using PG_GETARG_DATUM()?

Attached is a patch that fixes it, and brings it in line with all the
other PG_GETARG_FOO_P() macros.

- ilmari
-- 
"A disappointingly low fraction of the human race is,
 at any given time, on fire." - Stig Sandbeck Mathisen