This commit fixes two different issues in
`BLO_update_defaults_startup_blend`:
* No check were done whether a given 'paint mode' tool settings actually
exist or not before trying to 'update' it, leading to hard crash when
some did not.
* Only the first Scene in loaded 'startup' Main would be processed -
nothing prevents a `startup.blend` file to have more than one scene!
5f7f6b65f3