fix: memory leak in Statement#expanded_sql

from https://www.sqlite.org/c3ref/expanded_sql.html

> The strings returned by sqlite3_sql(P) and sqlite3_normalized_sql(P)
> are managed by SQLite and are automatically freed when the prepared
> statement is finalized. The string returned by
> sqlite3_expanded_sql(P), on the other hand, is obtained from
> sqlite3_malloc() and must be freed by the application by passing it
> to sqlite3_free().

ext/sqlite3/statement.c

@@ -622,10 +622,17 @@ static VALUE
 get_expanded_sql(VALUE self)
 {
     sqlite3StmtRubyPtr ctx;
+    char *expanded_sql;
+    VALUE rb_expanded_sql;
+
     TypedData_Get_Struct(self, sqlite3StmtRuby, &statement_type, ctx);
     REQUIRE_OPEN_STMT(ctx);
 
-    return rb_obj_freeze(SQLITE3_UTF8_STR_NEW2(sqlite3_expanded_sql(ctx->st)));
+    expanded_sql = sqlite3_expanded_sql(ctx->st);
+    rb_expanded_sql = rb_obj_freeze(SQLITE3_UTF8_STR_NEW2(expanded_sql));
+    sqlite3_free(expanded_sql);
+
+    return rb_expanded_sql;
 }
 
 void