archivers/pbzip2: fix stream decompression after libc++ 19 fix

The patch in bug 283655 to fix the pbzip2 build against libc++ 19 (which
finally removed std::char_traits<unsigned char>) had a problem where it
did not correctly check the bzip2 header. This only occurred when
decompressing from standard input, in the 'BZ2StreamScanner' mode, and
would then throw an unexpected "invalid file format" error.

PR: 283655
Reported by: wosch
MFH: 2024Q4