liblzma: Set all values in lzma_lz_encoder to NULL after allocation.

liblzma: Set all values in lzma_lz_encoder to NULL after allocation.

This is unlikely to be a bug in an existing application since it relies
on calling lzma_filters_update() on an LZMA1 encoder in the first place.
For instance, it does not affect xz because lzma_filters_update() can
only be used when encoding to the .xz format.

(based on commit 8191720eac950a5db89c4d33d6beea6316a49b19)