Fix segfalut that occurs when function is used in FROM clause

When a function is used in FROM, the parser makes a RangeFunction
node whose functions member is a list whose last element is NIL.
This caused segfault in foreach loop in _outListWith().

PRPARE statements in streaming-reaplication mode and INSERT/UPDATE
with SELECT in native-replication mode were affected. For example:

- prepare p as select * from generate_series(1,1);
- insert into tbl select now(), * from generate_series(1,1);

1 files changed, 1 insertions, 1 deletions

diff --git a/src/parser/outfuncs.c b/src/parser/outfuncs.c
index 6d702c5fc..417cb2cf0 100644
--- a/src/parser/outfuncs.c
+++ b/src/parser/outfuncs.c
@@ -290,7 +290,7 @@ static void _outListWith(String *str, List *node, char *op)
 			first = 1;
 		else
 		{
-			if (!IsA(lfirst(lc), A_Indices))
+			if (lfirst(lc) != NIL && !IsA(lfirst(lc), A_Indices))
 				string_append_char(str, op);
 		}
 		_outNode(str, lfirst(lc));