Fix type conversion in OP_STR_AT

Author: adazem009

src/engine/virtualmachine_p.cpp

@@ -647,7 +647,7 @@ do_list_get_item : {
     DISPATCH();
 
 do_str_at:
-    REPLACE_RET_VALUE(READ_REG(0, 2)->toUtf16()[READ_REG(1, 2)->toLong()], 2);
+    REPLACE_RET_VALUE(utf8::utf16to8(std::u16string({ READ_REG(0, 2)->toUtf16()[READ_REG(1, 2)->toLong() - 1] })), 2);
     FREE_REGS(1);
     DISPATCH();