Do not assert when plugging a vector which is too small.

It happens, especially at init that vector are null (of size 0).
In this case, avoid abrupt failure by just copying the available
data instead of aborting the whole software.

src/converter.hh

@@ -71,10 +71,16 @@ namespace dynamicgraph
   // Vector3
   SOT_TO_ROS_IMPL(specific::Vector3)
   {
-    assert (src.size () == 3);
-    dst.x =  src.elementAt (0);
-    dst.y =  src.elementAt (1);
-    dst.z =  src.elementAt (2);
+    if (src.size () > 0)
+      {
+	dst.x =  src.elementAt (0);
+	if (src.size () > 1)
+	  {
+	    dst.y =  src.elementAt (1);
+	    if (src.size () > 2)
+	      dst.z =  src.elementAt (2);
+	  }
+      }
   }
 
   ROS_TO_SOT_IMPL(specific::Vector3)