Reject missing main header in ArjArchiveInputStream.readMainHeader

[kali834x]

readHeader returns null when the basic header size field is zero, which is the arj end-of-archive marker, and readLocalFileHeader tests for that before using the result. readMainHeader does not, so an archive whose first header is the terminator reaches new ByteArrayInputStream(null) and throws NullPointerException out of a constructor that declares IOException; it escapes ArchiveStreamFactory the same way, since matches only needs the two signature bytes for the format to be detected. the whole input is 60 ea 00 00. throwing ArchiveException there matches the rest of readHeader's malformed-header handling, and the self-extracting path already rejects those bytes that way through findMainHeader, so only the streaming path changes. readHeader has two callers and this was the one missing the check.

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