API: Fix vector indexing

Description

Functions were always looking at index 0. Bounds check was also not checking for values greater than size. We weren't seeing issues because these functions don't have active callers.

Fixes compiler warnings on Windows build.

Motivation and Context

Looking through compiler warnings, and noticed these functions weren't implemented correctly.

How Has This Been Tested?

No breakpoint inspection since these functions aren't called.

Windows 7 - Stop recording hangs with and without change. Windows 10 - Recording works with and without change.

Windows compiler warnings are gone.

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

[x] My code has been run through clang-format.
[x] I have read the contributing document.
[x] My code is not on the master branch.
[x] The code has been tested.
[x] All commit messages are properly formatted and commits squashed where appropriate.

obsproject / obs-amd-encoder