In https://github.com/git-for-windows/git/issues/952, a complicated
scenario was described that leads to a segmentation fault in
cherry-pick.
It boils down to a certain code path involving a renamed file that is
dirty, for which `refresh_cache_entry()` returns `NULL`, and that
`NULL` not being handled properly.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>