Integer overflow caused by incorrect operation in struct file.

It seems like `b_unpack` function in `app/modules/struct.c` has an integer overflow vulnerability.
`size_t pos = luaL_optinteger(L, 3, 1) - 1;` statement cause integer overflow leading to out-of-bounds access.

It might be triggered with following script: `return struct.unpack('f', "xxxxxxxxxxxxx",-3)`

please check the PR for this issue. 



