libobs: Fix guards in obs.c

[kkartaltepe]

Fixes some unfortunately ordered guards that dereferenced obs before checking it is valid
Unifies some of the guard formatting

(totally didnt test this as I just noticed it when skimming. So im praying i didnt make a dumb typo, and relying on CI to check for it)

[jp9000]

Thank you, although this pull request is unnecessary. The reason is because of the following:

    struct obs_core_video *video = &obs->video;

In this statement, there is no actual dereferencing. When the obs pointer in this line is NULL, it does not crash; it will simply set the local variable, video, to the the offset of the obs_core::video member variable. See https://en.wikipedia.org/wiki/Offsetof -- that is why it's safe to have the if (!obs) return; statements after rather than before.

[kkartaltepe]

No worries, Thanks for taking a look.