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_encoder_mt.c

@@ -598,7 +598,7 @@ 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;