liblzma: Silence warnings from clang -Wconditional-uninitialized.

This is similar to 2ce4f36f17.
The actual initialization of the variables is done inside
mythread_sync() macro. Clang doesn't seem to see that
the initialization code inside the macro is always executed.

src/liblzma/common/stream_decoder_mt.c

@@ -1358,9 +1358,11 @@ stream_decode_mt(void *coder_ptr, const lzma_allocator *allocator,
 		// values after we read these as those changes can only be
 		// towards more favorable conditions (less memory in use,
 		// more in cache).
-		uint64_t mem_in_use;
-		uint64_t mem_cached;
-		struct worker_thread *thr = NULL; // Init to silence warning.
+		//
+		// These are initalized to silence warnings.
+		uint64_t mem_in_use = 0;
+		uint64_t mem_cached = 0;
+		struct worker_thread *thr = NULL;
 
 		mythread_sync(coder->mutex) {
 			mem_in_use = coder->mem_in_use;

src/liblzma/common/stream_encoder_mt.c

@@ -645,7 +645,7 @@ stream_encode_in(lzma_stream_coder *coder, const lzma_allocator *allocator,
 		}
 
 		if (block_error) {
-			lzma_ret ret;
+			lzma_ret ret = LZMA_OK; // Init to silence a warning.
 
 			mythread_sync(coder->mutex) {
 				ret = coder->thread_error;