Retry reading bag file messages without backslash in unit tests

[Achllle]

Description

This small fix addresses the case where the bag file specified for unit testing contains topics without a forward slash (/). If such a (legal) bag is used, currently the following error is returned:

    unable to parse value "/trajectory" (will be considered as string):
	list index out of range

The IndexError occurs because supplying the topic with forward slash returns the empty list. In the case that the user specified a non-existing topic, another IndexError will be raised as expected.

Testing

The following code produces a bag file that fails with current code and succeeds with this PR:

#!/usr/bin/env python
import rosbag

from trajectory_msgs.msg import JointTrajectory

bag = rosbag.Bag('test_some_state.bag', 'w')

try:
    jt = JointTrajectory()
    bag.write('trajectory', jt)  # succeeds when using '/trajectory

finally:
    bag.close()

[pschillinger]

Thanks for including this case!