Failed assertion when attempting to parse empty byte slice

[frewsxcv]

Found with cargo-fuzz via rust-fuzz/targets#95

extern crate minidump;

use std::io::Cursor;

fn main() {
    let cursor = Cursor::new(b"");
    let _ = minidump::Minidump::read(cursor);
}

thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `32`,
 right: `0`', /Users/corey/.cargo/git/checkouts/rust-minidump-836c4af26fbfd1e5/a4c7b38/src/iostuff.rs:28:4
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:68
             at src/libstd/sys_common/backtrace.rs:57
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:381
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:397
   4: std::panicking::begin_panic
             at src/libstd/panicking.rs:577
   5: std::panicking::begin_panic
             at src/libstd/panicking.rs:538
   6: std::panicking::try::do_call
             at src/libstd/panicking.rs:522
   7: minidump::iostuff::transmogrify
             at ./<panic macros>:7
   8: minidump::iostuff::read
             at /Users/corey/.cargo/git/checkouts/rust-minidump-836c4af26fbfd1e5/a4c7b38/src/iostuff.rs:40
   9: <minidump::minidump::Minidump<T>>::read
             at /Users/corey/.cargo/git/checkouts/rust-minidump-836c4af26fbfd1e5/a4c7b38/src/minidump.rs:1260
  10: foo::main
             at src/main.rs:7
  11: backtrace_vector_release
             at src/libpanic_unwind/lib.rs:101
  12: <std::thread::local::LocalKey<T>>::try_with
             at src/libstd/panicking.rs:459
             at src/libstd/rt.rs:58
  13: foo::main

[luser]

Interesting! This is an assertion failure, so it's not harmful, it's just not very nice. Minidump::read reads the minidump header here:
https://github.com/luser/rust-minidump/blob/a4c7b38c26fcaa90e4a86d9c0eb385d8ddbc7d3d/src/minidump.rs#L1260

which winds up calling this read method:
https://github.com/luser/rust-minidump/blob/a4c7b38c26fcaa90e4a86d9c0eb385d8ddbc7d3d/src/iostuff.rs#L37

which calls transmogrify, which has the failing assertion:
https://github.com/luser/rust-minidump/blob/a4c7b38c26fcaa90e4a86d9c0eb385d8ddbc7d3d/src/iostuff.rs#L28

...that code is not very good, certainly!

[luser]

I rewrote all the I/O to use the scroll crate on master and got rid of that awful transmogrify code. I changed the Minidump API slightly in the process so that Minidump::read now takes a Deref<[u8]>, so you can pass a Vec<u8> or &[u8] in there. I just added a test for this scenario and verified that it passes--it returns an error instead of panicing.

Thanks for the report!