Let shaper cope with low resolution system clock

We no longer rely on getting unique values from clock source, so we need
to handle cope with systems which does not have a microsecond resolution
on system clock (such as MS Windows)

src/shaper.erl

@@ -124,9 +124,13 @@ update(#maxrate{} = State, Size) ->
 	       true -> 0
 	    end,
     NextNow = p1_time_compat:system_time(micro_seconds) + Pause * 1000,
+    Div = case NextNow - State#maxrate.lasttime of
+        0 -> 1;
+        V -> V
+    end,
     {State#maxrate{lastrate =
 		       (State#maxrate.lastrate +
-			  1000000 * Size / (NextNow - State#maxrate.lasttime))
+			  1000000 * Size / Div)
 			 / 2,
 		   lasttime = NextNow},
      Pause}.