obs-vst: Modify bug in VSTPlugin::closeEditor()

[walker-WSH]

Description

VSTPlugin::closeEditor() can be invoked again from VSTPlugin::closeEditor(). Second closeEditor() is invoked from editorWidget->close() in first closeEditor(). In second func, editorWidget will be set with NULL, then deleteLater() in first func is using empty editorWidget

Motivation and Context

modify closeEditor()

How Has This Been Tested?

select VST-1, open editor UI; switch to VST-2

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.

[walker-WSH]

This PR may cause effEditClose can't be invoked. So I close this PR and will push another solution. https://github.com/obsproject/obs-vst/pull/100