branch: handle NULL pointers passed to git_branch_iterator_free()

Signed-off-by: Brodie Rao <brodie@sf.io>
config: handle NULL pointers passed to git_config_iterator_free()

Signed-off-by: Brodie Rao <brodie@sf.io>
odb: handle NULL pointers passed to git_odb_stream_free

Signed-off-by: Brodie Rao <brodie@sf.io>
oid: handle NULL pointers passed to git_oid_shorten_free()

Signed-off-by: Brodie Rao <brodie@sf.io>
refs: handle NULL pointers passed to git_reference_iterator_free()

Signed-off-by: Brodie Rao <brodie@sf.io>
util: handle NULL pointers passed to git_strarray_free()

Signed-off-by: Brodie Rao <brodie@sf.io>
Merge pull request #2039 from brodie/brodie/handle-null-on-free

Fix places in public free() functions where NULL pointers aren't handled