Bug fix: ff top_db is None then can not check `top_db>0` (#75)

* Bug fix: ff top_db is None then can not check `top_db>0`

* fix formatting

torchaudio/transforms.py

@@ -271,7 +271,9 @@ class SPEC2DB(object):
     """
     def __init__(self, stype="power", top_db=None):
         self.stype = stype
-        self.top_db = -top_db if top_db > 0 else top_db
+        if top_db is not None and top_db > 0:
+            top_db = -top_db
+        self.top_db = top_db
         self.multiplier = 10. if stype == "power" else 20.
 
     def __call__(self, spec):