hash: initialize m_dirty in BeesHashTable

It turns out we never set m_dirty's initial value.  This is not a
practical problem because 1) it's mostly harmless if m_dirty is spuriously
true, 2) we set it to true every time bees scans a data block, and 3)
the allocation happens early in startup when most memory allocations
are using zero-filled pages, so it's probably getting a false value at
construction in most cases.

valgrind complains about it, so it has to go.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees.h

@@ -459,7 +459,7 @@ private:
 	// Mutex/condvar for the writeback thread
 	mutex			m_dirty_mutex;
 	condition_variable	m_dirty_condvar;
-	bool			m_dirty;
+	bool			m_dirty = false;
 
 	// Mutex/condvar to stop
 	mutex			m_stop_mutex;