fix overwrite bug when adding symbol to dictionary

This bug ignored the tokens that were meant to be overwritten and appends them to the end of the dictionary symbols.

For example, a dictionary with 50K tokens that already has `<s>`, `</s>`, `<pad>` and `<unk>` with the #fairseq:overwrite tag will end up having 50004 tokens when loaded.

fairseq/data/dictionary.py

@@ -126,7 +126,7 @@ def unk_string(self, escape=False):
 
     def add_symbol(self, word, n=1, overwrite=False):
         """Adds a word to the dictionary"""
-        if word in self.indices and not overwrite:
+        if word in self.indices and overwrite:
             idx = self.indices[word]
             self.count[idx] = self.count[idx] + n
             return idx