Debugger: Only update executable from core file when needed

Conditions:
1. Not empty. It erases the existing executable for no reason.
2. Not Windows -> Windows doesn't produce core dumps, so the path inside it is
   unlikely to match anything on the host.

Change-Id: Ib3cd1ae278d8bc9f6ad326821319ca3deda44800
Reviewed-by: Thomas Otto <totto@zbh.uni-hamburg.de>
Reviewed-by: hjk <hjk121@nokiamail.com>