archivers/pbzip2: fix more issues after libc++ 19 patches

• Ensure the _bz2Header and _bz2HeaderZero members of BZ2StreamScanner do not contain an additional NUL byte.
• When comparing the magic header, compare from pHdr->begin() + hsp to pHdr->end(), making the code equivalent to what it was doing using std::basic_string::compare().

Submitted by: paparodeo@proton.me
PR: 283655
MFH: 2025Q1