zzip: fseeko: fix signedness mismatch with `file->avail`

zzip: fseeko: fix signedness mismatch with `file->avail`

While the `avail` field of `struct zzip_entry_file` is of unsigned type
`zzip_size_t`, we assign it to a local variable of `zzip_off_t` in
`zzip_entry_fopen`, which is of signed type. This causes the compiler to
emit a warning due to signedness when comparing it with
`sizeof(file->buffer)`.

Fix this by using the correct type for the local variable.