obs-vst: Update effect name even effect is unavailable

[walker-WSH]

Description

After choosing an unavailable effect, we should also update effect name. Otherwize the editor title will be previous effect's. But firstly we must check the magic number to make sure it is a vst2.x plugin.

Motivation and Context

fix bug about title of editor window

How Has This Been Tested?

steps: select a valid effect and open editor window; close editor window, choose an invalid effect, reopen editor window; check the editor title

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.
• [x] I have included updates to all appropriate documentation.