fix: panic when trying to iterate over an interface with zero endpoints

[alufers]

Some USB classes (UVC, UAC) provide alternate setting descriptors of interfaces with zero endpoints. Trying to call endpoint_descriptors on them caused a panic, because NULL was passed to slice::from_raw_parts. A branch was introduced to check for that case and create an iterator over an empty slice.

See: https://stackoverflow.com/a/49244874/2948315

Steps to reproduce the bug:

1. Have a device connected which has an interface (or interface altsetting) with zero endpoints. (for example: 17ef:482f Lenovo Lenovo 500 RGB Camera)
2. Run RUST_BACKTRACE=1 cargo run --example list_devices on the master branch
3. Observe panic:

Panic stacktrace

thread 'main' panicked at library/core/src/panicking.rs:155:5:
unsafe precondition(s) violated: slice::from_raw_parts requires the pointer to be aligned and non-null, and the total size of the slice not to exceed `isize::MAX`
stack backtrace:
   0: rust_begin_unwind
             at /rustc/b381d3ab27f788f990551100c4425bb782d26d76/library/std/src/panicking.rs:647:5
   1: core::panicking::panic_nounwind_fmt::runtime
             at /rustc/b381d3ab27f788f990551100c4425bb782d26d76/library/core/src/panicking.rs:110:18
   2: core::panicking::panic_nounwind_fmt
             at /rustc/b381d3ab27f788f990551100c4425bb782d26d76/library/core/src/panicking.rs:122:9
   3: core::panicking::panic_nounwind
             at /rustc/b381d3ab27f788f990551100c4425bb782d26d76/library/core/src/panicking.rs:155:5
   4: core::slice::raw::from_raw_parts::precondition_check
             at /rustc/b381d3ab27f788f990551100c4425bb782d26d76/library/core/src/intrinsics.rs:2647:21
   5: core::slice::raw::from_raw_parts
             at /rustc/b381d3ab27f788f990551100c4425bb782d26d76/library/core/src/slice/raw.rs:98:9
   6: rusb::interface_descriptor::InterfaceDescriptor::endpoint_descriptors
             at ./src/interface_descriptor.rs:106:27
   7: list_devices::list_devices
             at ./examples/list_devices.rs:70:42
   8: list_devices::main
             at ./examples/list_devices.rs:16:5
   9: core::ops::function::FnOnce::call_once
             at /rustc/b381d3ab27f788f990551100c4425bb782d26d76/library/core/src/ops/function.rs:250:5
             

Details and software versions:

• rusb master branch (commit 15c2910)
• libusb 1.0.27
• rustc 1.78.0-nightly (b381d3ab2 2024-02-12)
• Linux kernel 6.7.8

[alufers]

I think there seems to be a problem with the macos-latest check, it errors out on some internal libusb assertion. Same error in other PRs in this repo. Probably unrelated to the contents of this PR.

[a1ien]

LGTM. Thank you. Yes, macOS is experiencing issues due to a bug in libusb 1.0.26. I will try to find time to update libusb1-sys to use 1.0.27