liblzma: Prevent uninitialzed warning in mt stream encoder.

This change only impacts the compiler warning since it was impossible
for the wait_abs struct in stream_encode_mt() to be used before it was
initialized since mythread_condtime_set() will always be called before
mythread_cond_timedwait().

Since the mythread.h code is different between the POSIX and
Windows versions, this warning was only present on Windows builds.

Thanks to Arthur S for reporting the warning and providing an initial
patch.

src/liblzma/common/stream_encoder_mt.c

@@ -743,7 +743,7 @@ stream_encode_mt(void *coder_ptr, const lzma_allocator *allocator,
 
 		// These are for wait_for_work().
 		bool has_blocked = false;
-		mythread_condtime wait_abs;
+		mythread_condtime wait_abs = {};
 
 		while (true) {
 			mythread_sync(coder->mutex) {