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 - but *only* when
the 'Use External build' was enabled (presumably because having the files compiled in an external process somehow leads to the PSI not pointing to the physical *.scala files in some way.

http://blog.jetbrains.com/scala/2012/12/28/a-new-way-to-compile/