mingw: make `exit_process()` own the process handle on all paths

After "mingw: kill child processes in a gentler way", the ownership of the HANDLE passed to `exit_process()` and `terminate_process_tree()` is inconsistent. `terminate_process_tree()` always closes the handle; `exit_process()` closes it on success and on the terminate-tree fallback, but leaks it on the early return where GetExitCodeProcess() fails or reports the process is no longer STILL_ACTIVE. `mingw_kill()` compensated by closing the handle on its own error path, which is a double-close on every error path that does not hit that one leaky branch -- the callee has already closed the handle by then. Coverity flagged the resulting use-after-free as CID 1437238. Pin down the invariant that `exit_process()` and `terminate_process_tree()` own the handle from the call onward and close it on every return path; with that, the bogus close in `mingw_kill()` goes away. Assisted-by: Opus 4.7 Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>