edb crashes when adding a hardware breakpoint if a hardware breakpoint was attempted previously unsuccessfully

[AaronOpfer]

Steps to repro:

• attach edb to a process
  • resume so that the process is running
  • attempt to set a hardware read/write breakpoint in the memory dump view
  • see that it fails with an error suggesting you pause the process
  • pause the process
  • reattempt to set the hardware read/write breakpoint in the memory dump view

Expected: breakpoint is set successfully

Actual: edb crashes with the following in the terminal

/home/aopfer/gits/edb-debugger/src/DebugEventHandlers.cpp:40: void DebugEventHandlers::add(IDebugEventHandler*): Fatal error: the same event handler is added twice

[eteran]

Ok, should be an easy fix. Thanks!