diff: fix a possible null pointer dereference

diff: fix a possible null pointer dereference

The condition in the ternary operator was wrong, hence the wrong char
pointer could be used as the parameter for show_submodule_summary.
one->path may be null, but we definitely need a non null path given
to the function.

Signed-off-by: Stefan Beller <redacted>
Acked-By: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>