diskfile: Add some argument validation

Either all or none of account, container, and object should be provided.
If we get some but not all, that's indicating some kind of a coding bug;
there's a chance it may be benign, but it seems safer to fail early and
loudly.

Change-Id: Ia9a0ac28bde4b5dcbf6e979c131e61297577c120
Related-Change: Ic2e29474505426dea77e178bf94d891f150d851b

swift/obj/diskfile.py

@@ -2231,6 +2231,10 @@ class BaseDiskFile(object):
             self._account = account
             self._container = container
             self._obj = obj
+        elif account or container or obj:
+            raise ValueError(
+                'Received a/c/o args %r, %r, and %r. Either none or all must '
+                'be provided.' % (account, container, obj))
         else:
             # gets populated when we read the metadata
             self._name = None