handle UnboundLocalError in ar_spelling_checker

You forgot to initialize tmp_corrections, when` if not word in vocab:` becomes `false`, exception occur at line 127:
`UnboundLocalError: local variable 'tmp_corrections' referenced before assignment`

app.py

@@ -123,6 +123,7 @@ def ar_spelling_checker(text):
   result = {}
 
   for word in word_l:
+    tmp_corrections = []
     if not word in vocab:
       tmp_corrections = get_corrections(word, vocab)
       if len(tmp_corrections) == 0: