Avoid the use of an uninitialized value when the write buffers have
not been initialized.  A recent change to avoid the use of strm->
next_in in order to resolve some const conflicts added the use of
state->in in its place.  This patch avoids the use of state->in
when it is not initialized.  Nothing bad would actually happen,
since two variables set to the same unintialized value are
subtracted.  However valgrind was rightly complaining.  So this
fixes that.