Reinstate strictness of int4_avg_combine

The "combine" function for the int4 sum/avg aggregate functions is
backported to Greenplum 6.0 in commit 313cef6e (from
postgres/postgres@11c8669c0cc) but we made the inevitable omission of
setting the "strictness" of int4_avg_combine to false. This by itself is
harmless as the actual function body *does* guard against NULL input,
but it also prohibits a whole host of optimizations when the executor
and planner can detect NULL input early on and short-circuit the
execution. Oops.

This patch flips the `proisstrict` flag back to true for
int4_avg_combine.

Backpatch to 6X_STABLE.

src/include/catalog/catversion.h

@@ -56,6 +56,6 @@
  */
 
 /*							3yyymmddN */
-#define CATALOG_VERSION_NO	301905081
+#define CATALOG_VERSION_NO	301905101
 
 #endif

src/include/catalog/pg_proc.h

@@ -2543,7 +2543,7 @@ DATA(insert OID = 2786 (  int8_avg_serialize    PGNSP PGUID 12 1 0 0 0 f f f f t
 DESCR("aggregate serial function");
 DATA(insert OID = 2787 (  int8_avg_deserialize    PGNSP PGUID 12 1 0 0 0 f f f f t f i 2 0 2281 "17 2281" _null_ _null_ _null_ _null_ int8_avg_deserialize _null_ _null_ _null_ ));
 DESCR("aggregate deserial function");
-DATA(insert OID = 3324 (  int4_avg_combine    PGNSP PGUID 12 1 0 0 0 f f f f f f i 2 0 1016 "1016 1016" _null_ _null_ _null_ _null_ int4_avg_combine _null_ _null_ _null_ ));
+DATA(insert OID = 3324 (  int4_avg_combine    PGNSP PGUID 12 1 0 0 0 f f f f t f i 2 0 1016 "1016 1016" _null_ _null_ _null_ _null_ int4_avg_combine _null_ _null_ _null_ ));
 DESCR("aggregate combine function");
 DATA(insert OID = 3178 (  numeric_sum	   PGNSP PGUID 12 1 0 0 0 f f f f f f i 1 0 1700 "2281" _null_ _null_ _null_ _null_ numeric_sum _null_ _null_ _null_ ));
 DESCR("aggregate final function");