liblzma: Prevent uninitialzed warning in mt stream encoder.

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.

(cherry picked from commit 1155471651ad456c5f90aee6435931fae65682bf)