Check `base` property before using it

OverrideModule's `base` property should point a valid
module(OverridableModule).

It should be checked before using it.

Bug: 159503211
Test: add `override_apex` without `base`
      m emits error, not panic.
Change-Id: I1d7c9395cc9a7461e317dd4b6ddbec3a9e68e9bf
(cherry picked from commit 3851b0c1fe8e8ffd3d30b4b8a5289f39a2c98270)