Fix MemoryMappedFileInputStream.read() returning sign-extended values for bytes over 0x7F

[lenamonj]

MemoryMappedFileInputStream.read() returned Short.toUnsignedInt(buffer.get()), which widens the byte to a short with sign extension before masking, so every byte from 0x80 to 0xFF came back as 0xFF80 to 0xFFFF instead of 128 to 255. InputStream.read() promises 0 to 255, and a caller copying byte by byte writes the wrong values out. The existing testReadSingleByte casts each result back to byte before comparing, which hides the difference.

The change masks with & 0xFF, as the other single-byte read() implementations in this package do.

Test: testReadSingleByteIsUnsigned, red on master and green with the change. Default Maven goal green.

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