merge-recursive: handle NULL in add_cacheinfo() correctly

merge-recursive: handle NULL in add_cacheinfo() correctly

1335d76e45 ("merge: avoid "safer crlf" during recording of merge
results", 2016-07-08) tried to split make_cache_entry() call made
with CE_MATCH_REFRESH into a call to make_cache_entry() without one,
followed by a call to add_cache_entry(), refresh_cache() and another
add_cache_entry() as needed.  However the conversion was botched in
that it forgot that refresh_cache() can return NULL, which was
handled correctly in make_cache_entry() but in the updated code.

This fixes https://github.com/git-for-windows/git/issues/952

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>