lzx_compress: fix corruption with long literal run

lzx_compress: fix corruption with long literal run

The last round of updates to the LZX compressor made it start being able
to use larger blocks, up to ~100KB.  Unfortunately it was overlooked
that this allows literal runs > 65535 bytes while in one place the
length of a literal run was still being stored in a u16.  Therefore, on
incompressible input data this could be wrapped around, causing
incorrect compression.  Fix this by enlarging the variable.