fixup! download: verify checksum of downloaded data

slapos/libnetworkcache.py

@@ -90,9 +90,9 @@ class CheckResponse(object):
 
   def read(self, amt=None):
     r = self._response.read(amt)
-    if r:
-      self._sha512sum.update(r)
-    elif amt != 0 and self._expected != self._sha512sum.hexdigest():
+    self._sha512sum.update(r)
+    if (not r if amt else amt is None) and \
+        self._expected != self._sha512sum.hexdigest():
       raise NetworkcacheException(
             'Failed to download data to SHACACHE Server: invalid checksum.')
     return r