libobs: Check for extension validity in os_generate_formatted_filename

In some cases (ffmpeg_mux), the extension and format values are a
direct passthrough from arbitrary data. Instead of always postfixing
a `.` at the end of a generated path if the extension is invalid,
don't postfix the `.` at all.

Before: `my_formatted_string.`
After: `my_formatted_string`
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