Ensure zero initialization of PosVelAccState::time_from_start

[miguelprada]

On a very specific setup using joint_trajectory_controller, I started to get strange random crashes with:

terminate called after throwing an instance of 'std::runtime_error'
  what():  Duration is out of dual 32-bit range

Digging a bit, it seems that the program terminates here because of an uninitialized PosVelAccState::time_from_start.

Further digging showed that the problematic state had been default constructed here, and checking the default constructor I can see that time_from_start is never initialized.

This seems to fix the problem for me, although it's difficult to confirm because the error is hard to reproduce reliably.

[miguelprada]

Closing as duplicate of #577. I checked the existing issues but somehow missed that open PR 🤦