Debugger: Fix engine state after "unreasonable jump requests"

After attempting to jump to an inaccessible location, the
engine was still expecting a ^running, while in fact GDB
was stopped. Add the right state transition.

Change-Id: I69aec785a1e9d2a23771f74ae35064ae61405fa5
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>