Fix PG_GETARG_SEG_P() definition.

DatumGetPointer() takes a Datum argument, not a pointer.
This is cosmetic given the current definitions of the
underlying macros, but it's still formally a type violation.

Bug was introduced in commit 389bb281, but there seems
no need to back-patch.

Dagfinn Ilmari Mannsåker

Discussion: https://postgr.es/m/d8jlfsxq3a0.fsf@dalvik.ping.uio.no

contrib/seg/seg.c

@@ -19,7 +19,7 @@
 
 
 #define DatumGetSegP(X) ((SEG *) DatumGetPointer(X))
-#define PG_GETARG_SEG_P(n) DatumGetSegP(PG_GETARG_POINTER(n))
+#define PG_GETARG_SEG_P(n) DatumGetSegP(PG_GETARG_DATUM(n))
 
 
 /*