diff_tform: fix potential NULL pointer access

diff_tform: fix potential NULL pointer access

The `normalize_find_opts` function in theory allows for the
incoming diff to have no repository. When the caller does not
pass in diff find options or if the GIT_DIFF_FIND_BY_CONFIG value
is set, though, we try to derive the configuration from the
diff's repository configuration without first verifying that the
repository is actually set to a non-NULL value.

Fix this issue by explicitly checking if the repository is set
and if it is not, fall back to a default value of
GIT_DIFF_FIND_RENAMES.