Ensure file exists before browsing for subtitles

Prevents an NPE from ```getParentFile()```when ```file``` is really a url. The context is a call from the ```isBravia``` section of [DLNAResource.getString (#L1586)](https://github.com/UniversalMediaServer/UniversalMediaServer/blob/master/src/main/java/net/pms/dlna/DLNAResource.java#L1586) which assumes the video can only be a real file.