Fix for ScalaLanguageLevel crashing with External build compiler

`ScalaLanguageLevel.getLanguageLevel(element)` would crash if `element.getContainingFile.getVirtualFile` was null. This would pretty much kill any Completion/Highlighting processes that touched it, as in this bug report against Completion:

http://youtrack.jetbrains.com/issue/SCL-5108

I think getVirtualFile() is supposed to return null iff the file exists only in memory, however this was occurring for me with Scala 0.7.62/IU-123.94 on concrete .scala source files.

I previously thought that this was occurring only when 'Use External build' was enabled, but in fact the problem with getVirtualFile() returning null seems independent of that.

http://blog.jetbrains.com/scala/2012/12/28/a-new-way-to-compile/