Prevent segfault in expand_tuple with no missing values

Commit 16828d5c forgot to check that it had a set of missing values
before trying to retrieve a value from it.

An additional query to add coverage for this code is added to the
regression test.

Per bug report from Andreas Seltenreich.

src/backend/access/common/heaptuple.c

@@ -981,7 +981,7 @@ expand_tuple(HeapTuple *targetHeapTuple,
 
 		Form_pg_attribute attr = TupleDescAttr(tupleDesc, attnum);
 
-		if (attrmiss[attnum].ammissingPresent)
+		if (attrmiss && attrmiss[attnum].ammissingPresent)
 		{
 			fill_val(attr,
 					 nullBits ? &nullBits : NULL,

src/test/regress/expected/fast_default.out

@@ -505,6 +505,41 @@ SELECT comp();
  Unchanged
 (1 row)
 
+-- query to exercise expand_tuple function
+CREATE TABLE t1 AS
+SELECT 1::int AS a , 2::int AS b
+FROM generate_series(1,20) q;
+ALTER TABLE t1 ADD COLUMN c text;
+SELECT a,
+       stddev(cast((SELECT sum(1) FROM generate_series(1,20) x) AS float4))
+          OVER (PARTITION BY a,b,c ORDER BY b)
+       AS z
+FROM t1;
+ a | z 
+---+---
+ 1 | 0
+ 1 | 0
+ 1 | 0
+ 1 | 0
+ 1 | 0
+ 1 | 0
+ 1 | 0
+ 1 | 0
+ 1 | 0
+ 1 | 0
+ 1 | 0
+ 1 | 0
+ 1 | 0
+ 1 | 0
+ 1 | 0
+ 1 | 0
+ 1 | 0
+ 1 | 0
+ 1 | 0
+ 1 | 0
+(20 rows)
+
+DROP TABLE t1;
 DROP TABLE T;
 DROP FUNCTION set(name);
 DROP FUNCTION comp();

src/test/regress/sql/fast_default.sql

@@ -347,6 +347,20 @@ SELECT c_text FROM T WHERE c_int = -1;
 
 SELECT comp();
 
+-- query to exercise expand_tuple function
+CREATE TABLE t1 AS
+SELECT 1::int AS a , 2::int AS b
+FROM generate_series(1,20) q;
+
+ALTER TABLE t1 ADD COLUMN c text;
+
+SELECT a,
+       stddev(cast((SELECT sum(1) FROM generate_series(1,20) x) AS float4))
+          OVER (PARTITION BY a,b,c ORDER BY b)
+       AS z
+FROM t1;
+
+DROP TABLE t1;
 DROP TABLE T;
 DROP FUNCTION set(name);
 DROP FUNCTION comp();