Bug 1016473 - Gracefully handle bad zip data gotten from storage. r=mshal

cache.py

@@ -4,7 +4,7 @@
 
 import shutil
 from cStringIO import StringIO
-from zipfile import ZipFile, ZIP_DEFLATED
+from zipfile import ZipFile, ZIP_DEFLATED, BadZipfile
 
 
 class CacheData(object):
@@ -22,7 +22,10 @@ class CacheData(object):
     def __init__(self, data=None):
         self._data = StringIO(data) if data else StringIO()
         self._obj = {}
-        self._zip = ZipFile(self._data, 'r' if data else 'w', ZIP_DEFLATED)
+        try:
+            self._zip = ZipFile(self._data, 'r' if data else 'w', ZIP_DEFLATED)
+        except BadZipfile:
+            self._zip = ZipFile(self._data, 'w', ZIP_DEFLATED)
 
     def __getitem__(self, key):
         if key not in self._obj: