Fix crash in nilmdb-fsck if there are zero intervals, etc.

nilmdb/fsck/fsck.py

@@ -59,6 +59,8 @@ def retry_if_raised(exc, message = None, max_retries = 100):
 
 class Progress(object):
     def __init__(self, maxval):
+        if maxval == 0:
+            maxval = 1
         self.bar = progressbar.ProgressBar(
             maxval = maxval,
             widgets = [ progressbar.Percentage(), ' ',