Urbano Plugin throws exception when topic not found.

[tian2992]

Urbano throws error when called with a not-found parameter.

Unhandled Error
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 524, in __bootstrap
    self.__bootstrap_inner()
  File "/usr/lib/python2.7/threading.py", line 551, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 504, in run
    self.__target(*self.__args, **self.__kwargs)
--- <exception caught here> ---
  File "/home/tian/tio_chema/venv/local/lib/python2.7/site-packages/twisted/python/threadpool.py", line 172, in _worker
    result = context.call(ctx, function, *args, **kwargs)
  File "/home/tian/tio_chema/venv/local/lib/python2.7/site-packages/twisted/python/context.py", line 118, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File "/home/tian/tio_chema/venv/local/lib/python2.7/site-packages/twisted/python/context.py", line 81, in callWithContext
    return func(*args,**kw)
  File "/home/tian/tio_chema/plugins/urbano.py", line 35, in execute
    respuesta = definiciones[ randint( 0, len( definiciones ) - 1 ) ]
  File "/usr/lib/python2.7/random.py", line 241, in randint
    return self.randrange(a, b+1)
  File "/usr/lib/python2.7/random.py", line 217, in randrange
    raise ValueError, "empty range for randrange() (%d,%d, %d)" % (istart, istop, width)
exceptions.ValueError: empty range for randrange() (0,0, 0)