Fixed pgAdmin crash issue

Hi Dave,

Please find the patch file in attachment for below described issue in linux.

*Steps to reproduce:-*
*
*
1. Open pgAdmin and go to database node of larger size.
2. Right Click on that database and go to "Backup"
3. Give the Filename = "test" and Format = "Custom".
4. Click on "Backup" button and immediately click "Cancel" button to cancel
the
operation.
5. pgAdmin will crash.

*Findings:-*
*
*
As we are creating the new process with "wxExecute" to create the Backup
and user immediately press on "Cancel" then we should first kill the
process which is created and proper way to delete the process is once we
get the callback of "EndProcess" not in Abort() method so changed it
accordingly.

Kindly let me know for any modification.

Thanks,
Neel Patel