Panic due to integer wrapping

[evanrichter]

The attached img file will cause the following subtraction to overflow:

rust-fatfs/src/file.rs

Line 283 in 4892fb1

let bytes_left_in_cluster = (cluster_size - offset_in_cluster) as usize;

img.zip

The application code can be seen here: https://github.com/evanrichter/rust-fatfs/blob/16f52f4d64c6d54ca9290f66e5167e1f2ddad552/fuzz/fuzz_targets/fs_load.rs