Deparser: Add support for COALESCE and other expressions in LIMIT clause

We've previously supported "c_expr" being used in a LIMIT clause, but
its actually more correct to treat this as "a_expr", which is a broader
set of functions, including COALESCE.

Note that the previous handling (for "c_expr") is still correct for
FETCH FIRST (expr) ROWS WITH TIES, see the "select_fetch_first_value"
in the Postgres grammar.

In passing fix two new warnings in the deparser caused by a recent code
addition.

src/postgres_deparse.c

@@ -2267,9 +2267,11 @@ static void deparseSelectStmt(StringInfo str, SelectStmt *stmt)
 
 		if (IsA(stmt->limitCount, A_Const) && castNode(A_Const, stmt->limitCount)->isnull)
 			appendStringInfoString(str, "ALL");
-		else
+		else if (stmt->limitOption == LIMIT_OPTION_WITH_TIES)
 			deparseCExpr(str, stmt->limitCount);
-
+		else
+			deparseExpr(str, stmt->limitCount);
+
 		appendStringInfoChar(str, ' ');
 
 		if (stmt->limitOption == LIMIT_OPTION_WITH_TIES)
@@ -4899,13 +4901,15 @@ static void deparseCreateFunctionStmt(StringInfo str, CreateFunctionStmt *create
 		/* RETURN or BEGIN ... END
 		 */
 		if (IsA(create_function_stmt->sql_body, ReturnStmt))
+		{
 			deparseReturnStmt(str, castNode(ReturnStmt, create_function_stmt->sql_body));
+		}
 		else
 		{
 			appendStringInfoString(str, "BEGIN ATOMIC ");
-			if (linitial(create_function_stmt->sql_body) != NULL)
+			if (IsA(create_function_stmt->sql_body, List), linitial((List *) create_function_stmt->sql_body) != NULL)
 			{
-				List *body_stmt_list = castNode(List, linitial(create_function_stmt->sql_body));
+				List *body_stmt_list = castNode(List, linitial((List *) create_function_stmt->sql_body));
 				foreach(lc, body_stmt_list)
 				{
 					if (IsA(lfirst(lc), ReturnStmt))

test/deparse_tests.c

@@ -397,6 +397,7 @@ const char* tests[] = {
   "CREATE PROCEDURE do_nothing() LANGUAGE sql BEGIN ATOMIC END",
   "CREATE PROCEDURE returns_one() LANGUAGE sql BEGIN ATOMIC RETURN 1; END",
   "CREATE PROCEDURE updates_and_returns_one() LANGUAGE sql BEGIN ATOMIC UPDATE tbl SET a = 1; RETURN 1; END",
+  "SELECT 1 FROM tbl LIMIT COALESCE($1, $2)",
 };
 
 size_t testsLength = __LINE__ - 4;