Fix function UnicityTable::push_back (issue #3925)

mftraining crashed because the returned value was 1 instead of 0
for the first call of UnicityTable::push_back.

Signed-off-by: Stefan Weil <sw@weilnetz.de>

src/ccutil/unicity_table.h

@@ -73,8 +73,7 @@ class UnicityTable {
   int push_back(T object)  {
     auto idx = get_index(object);
     if (idx == -1) {
-      table_.push_back(object);
-      idx = size();
+      idx = table_.push_back(object);
     }
     return idx;
   }