dedupe: Skip extents with insufficient duplicates (#365)

Add safety check in push_extents() to prevent queuing work items
with less than 2 duplicates. This prevents assertion failures in
dedupe_extent_list() when the duplicate count drops below 2 between
the initial duplicate detection and worker thread processing.

Fixes: #341

run_dedupe.c

@@ -561,11 +561,16 @@ static int push_extents(struct results_tree *res)
 		 * around the rbtree code here so rb_erase doesn't
 		 * change the tree underneath us.
 		 */
-
 		g_mutex_lock(&mutex);
 		node = rb_next(node);
 		g_mutex_unlock(&mutex);
 
+		if (dext->de_num_dupes < 2) {
+			qprintf("Skipping extent - insufficient duplicates (%u)\n",
+				   dext->de_num_dupes);
+			continue;
+		}
+
 		g_thread_pool_push(dedupe_pool, dext, &err);
 		if (err) {
 			eprintf("Fatal error while deduping: %s\n",