Avoid allocating in ReadExact when insufficient data

[fbergero]

Summary

• Adds a seek-based bounds check in DekuReader<ReadExact> for Vec<u8> to verify the underlying reader has enough bytes before allocating the output buffer
• Uses reader.as_mut() to seek on the inner reader directly, avoiding side effects on Reader's bits_read and leftover state
• Returns DekuError::Incomplete early when data is insufficient

Problem

The current implementation unconditionally allocates and zero-fills a buffer of exact.0 bytes:

let mut bytes = alloc::vec![0x00; exact.0];
let _ = reader.read_bytes(exact.0, &mut bytes, Order::Lsb0)?;

If exact.0 comes from untrusted input (e.g., a length-prefixed protocol), a malformed value can cause a huge allocation that gets immediately discarded when read_bytes fails. This is a DoS vector for any binary parser handling network data.

Fix

Three cheap seeks (get position, seek to end, restore) to compute remaining bytes, then early-return Incomplete if insufficient. Zero overhead for the happy path beyond those seeks.

Tests

• Unit tests for happy path, partial reads, empty input, zero-length, and error cases
• Allocation-counting test using alloc_counter (existing dev-dependency) that proves (0, 0, 0) allocations on the error path with ReadExact(1_000_000) on a 3-byte buffer

Test plan

• All existing vec tests pass (32/32)
• New unit tests pass (6/6)
• Allocation counting test confirms zero allocations on error path

🤖 Generated with Claude Code

[fbergero]

@sharksforarms Hey idk what's your AI policy for contributions, but feel free to drop this PR if it's against the policy