when WHEEL file is corrupted, set wheeldata to None instead of metada…

…ta (#23)

wheelfile.py

@@ -1671,7 +1671,7 @@ def _read_distinfo(self):
             wheeldata = self.zipfile.read(self._distinfo_path('WHEEL'))
             self.wheeldata = WheelData.from_str(wheeldata.decode('utf-8'))
         except Exception:
-            self.metadata = None
+            self.wheeldata = None
 
         try:
             record = self.zipfile.read(self._distinfo_path('RECORD'))