sql: fix possible null dereference in sql_expr_coll()

Some built-in functions can accept different number of arguments.
Check of argument count for such functions takes place right before
its execution. So it is possible that expression-list representing
arguments for built-in function is NULL. On the other hand, in
sql_expr_coll() (which returns collation of expression) it is assumed
that if function features SQL_FUNC_DERIVEDCOLL flag (it implies that
resulting collation depends on collation of arguments) then it has at
least one argument. The last assumption is wrong considering for example
SUBSTR() function: it may have 1 or 2 arguments, so check of argument
count doesn't occur during compilation. Hence, if it is required to
calculate collation for SUBSTR() function and there's no arguments,
Tarantool crashes due to null-dereference.
This patch fixes this bug with one additional check in sql_expr_coll().

src/box/sql/expr.c

@@ -332,7 +332,8 @@ sql_expr_coll(Parse *parse, Expr *p, bool *is_explicit_coll, uint32_t *coll_id,
 				sql_func_by_signature(p->u.zToken, arg_count);
 			if (func == NULL)
 				break;
-			if (sql_func_flag_is_set(func, SQL_FUNC_DERIVEDCOLL)) {
+			if (sql_func_flag_is_set(func, SQL_FUNC_DERIVEDCOLL) &&
+			    arg_count > 0) {
 				/*
 				 * Now we use quite straightforward
 				 * approach assuming that resulting

test/sql/collation.result

@@ -292,6 +292,15 @@ box.execute("SELECT * FROM t WHERE a COLLATE \"binary\" = c COLLATE \"unicode\";
 - null
 - Illegal mix of collations
 ...
+-- Make sure that using function featuring variable arguemnts
+-- length  and resulting collation which depends on arguments
+-- is processed correctly.
+--
+box.execute("SELECT * FROM t WHERE a COLLATE \"binary\" = substr();")
+---
+- null
+- 'Wrong number of arguments is passed to SUBSTR(): expected 1 or 2, got 0'
+...
 -- Compound queries perform implicit comparisons between values.
 -- Hence, rules for collations compatibilities are the same.
 --

test/sql/collation.test.lua

@@ -80,6 +80,11 @@ box.execute("SELECT * FROM t WHERE b = c;")
 box.execute("SELECT * FROM t WHERE b COLLATE \"binary\" = c;")
 box.execute("SELECT * FROM t WHERE a = c;")
 box.execute("SELECT * FROM t WHERE a COLLATE \"binary\" = c COLLATE \"unicode\";")
+-- Make sure that using function featuring variable arguemnts
+-- length  and resulting collation which depends on arguments
+-- is processed correctly.
+--
+box.execute("SELECT * FROM t WHERE a COLLATE \"binary\" = substr();")
 
 -- Compound queries perform implicit comparisons between values.
 -- Hence, rules for collations compatibilities are the same.