liblzma: Initialize lzma_lz_encoder pointers with NULL.

This fixes the recent change to lzma_lz_encoder that used memzero
instead of the NULL constant. On some compilers the NULL constant
(always 0) may not equal the NULL pointer (this only needs to guarentee
to not point to valid memory address).

Later code compares the pointers to the NULL pointer so we must
initialize them with the NULL pointer instead of 0 to guarentee
code correctness.

src/liblzma/lz/lz_encoder.c

@@ -564,7 +564,11 @@ lzma_lz_encoder_init(lzma_next_coder *next, const lzma_allocator *allocator,
 		next->update = &lz_encoder_update;
 		next->set_out_limit = &lz_encoder_set_out_limit;
 
-		memzero((&coder->lz), sizeof(lzma_lz_encoder));
+		coder->lz.coder = NULL;
+		coder->lz.code = NULL;
+		coder->lz.end = NULL;
+		coder->lz.options_update = NULL;
+		coder->lz.set_out_limit = NULL;
 
 		// mf.size is initialized to silence Valgrind
 		// when used on optimized binaries (GCC may reorder