larry.keep_label crashes when there is nothing to keep

[kwgoodman]

>> y = la.larry([1, 2])
>> y.keep_label('==', 99, axis=0) # <--- Nothing to keep
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)

/ba/devel/larry/<ipython console> in <module>()

/ba/devel/larry/la/deflarry.pyc in keep_label(self, op, value, axis)
   1730 cmd = cmd + op + ' value]'
   1731 idxlabel = eval(cmd)
-> 1732 idx, label = zip(*idxlabel)
   1733 y.label[axis] = list(label)
   1734 index = [slice(None,None,None)] * self.ndim

ValueError: need more than 0 values to unpack