Fix failure to cover scalar-vs-rowtype cases in exec_stmt_return().

In commit 9e3ad1aac5 I modified plpgsql
to use exec_stmt_return's simple-variables fast path in more cases.
However, I overlooked that there are really two different return
conventions in use here, depending on whether estate->retistuple is true,
and the existing fast-path code had only bothered to handle one of them.
So trying to return a scalar in a function returning composite, or vice
versa, could lead to unexpected error messages (typically "cache lookup
failed for type 0") or to a null-pointer-dereference crash.

In the DTYPE_VAR case, we can just throw error if retistuple is true,
corresponding to what happens in the general-expression code path that was
being used previously.  (Perhaps someday both of these code paths should
attempt a coercion, but today is not that day.)

In the REC and ROW cases, just hand the problem to exec_eval_datum()
when not retistuple.  Also clean up the ROW coding slightly so it looks
more like exec_eval_datum().

The previous commit also caused exec_stmt_return_next() to be used in
more cases, but that code seems to be OK as-is.

Per off-list report from Serge Rielau.  This bug is new in 9.5 so no need
to back-patch.

src/test/regress/expected/plpgsql.out

@@ -4001,6 +4001,38 @@ $$ language plpgsql;
 select compos();
 ERROR:  cannot return non-composite value from function returning composite type
 CONTEXT:  PL/pgSQL function compos() line 3 at RETURN
+-- RETURN variable is a different code path ...
+create or replace function compos() returns compostype as $$
+declare x int := 42;
+begin
+  return x;
+end;
+$$ language plpgsql;
+select * from compos();
+ERROR:  cannot return non-composite value from function returning composite type
+CONTEXT:  PL/pgSQL function compos() line 4 at RETURN
+drop function compos();
+-- test: invalid use of composite variable in scalar-returning function
+create or replace function compos() returns int as $$
+declare
+  v compostype;
+begin
+  v := (1, 'hello');
+  return v;
+end;
+$$ language plpgsql;
+select compos();
+ERROR:  invalid input syntax for integer: "(1,hello)"
+CONTEXT:  PL/pgSQL function compos() while casting return value to function's return type
+-- test: invalid use of composite expression in scalar-returning function
+create or replace function compos() returns int as $$
+begin
+  return (1, 'hello')::compostype;
+end;
+$$ language plpgsql;
+select compos();
+ERROR:  invalid input syntax for integer: "(1,hello)"
+CONTEXT:  PL/pgSQL function compos() while casting return value to function's return type
 drop function compos();
 drop type compostype;
 --

src/test/regress/sql/plpgsql.sql

@@ -3248,6 +3248,39 @@ $$ language plpgsql;
 
 select compos();
 
+-- RETURN variable is a different code path ...
+create or replace function compos() returns compostype as $$
+declare x int := 42;
+begin
+  return x;
+end;
+$$ language plpgsql;
+
+select * from compos();
+
+drop function compos();
+
+-- test: invalid use of composite variable in scalar-returning function
+create or replace function compos() returns int as $$
+declare
+  v compostype;
+begin
+  v := (1, 'hello');
+  return v;
+end;
+$$ language plpgsql;
+
+select compos();
+
+-- test: invalid use of composite expression in scalar-returning function
+create or replace function compos() returns int as $$
+begin
+  return (1, 'hello')::compostype;
+end;
+$$ language plpgsql;
+
+select compos();
+
 drop function compos();
 drop type compostype;