CVE-2021-29934: Out of bounds read in uu_od

The vulnerability stemmed from PartialReader's read implementation using uninitialized memory buffers. The original code (pre-patch) used Vec::with_capacity followed by unsafe set_len to create a buffer, then passed slices of this uninitialized memory to Read operations. This violates Rust's safety guarantees as user-provided Read implementations could observe uninitialized memory. The patch replaced this with a zero-initialized array and changed read_exact to read with proper error handling, confirming the vulnerability was in this specific function.