autopilot: fixme division by zero

I don't understand the code at that location, but it can raise
a division by zero in testing environment where no real graph
was accessible because it couldn't be downloaded.

autopilot/lib_autopilot.py

@@ -243,6 +243,7 @@ class Autopilot():
             path_pdf[node] = path_sum
 
         s = sum(path_pdf.values())
+        # FIXME: next line can raise a division by zero
         path_pdf = {k: v / s for k, v in path_pdf.items()}
         self.__logger.info(
             "DECREASE DIAMETER: probability density function created")