fix: cobs accumulator out-of-bounds index when data is 1 byte too long

[jedrzejboczar]

Currently there is an off-by-one error in bounds checking which may lead to panic! in self.extend_unchecked().

When calling:

let (take, release) = input.split_at(n + 1);

the take slice has length of n + 1 (slice::split_at), but currently the if condition checks for size n.

I added a test case that demonstrates the issue using CobsAccumulator<11> and feeding it with a 12-byte message. Without the fix this will panic:

thread 'accumulator::extend_unchecked_in_bounds_test' panicked at 'range end index 12 out of range for slice of length 11', library/core/src/slice/index.rs:73:5

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[jamesmunns]

Thank you!