Fixed a crash in liblzma.

liblzma tries to avoid useless free()/malloc() pairs in
initialization when multiple files are handled using the
same lzma_stream. This didn't work with filter chains
due to comparison of wrong pointers in lzma_next_coder_init(),
making liblzma think that no memory reallocation is needed
even when it actually is.

Easy way to trigger this bug is to decompress two files with
a single xz command. The first file should have e.g. x86+LZMA2
as the filter chain, and the second file just LZMA2.