IZ #167358: NPE @ PHPHintsProvider.computeHints()

Impact: we don't know the steps to reproduce, but there was 60 
duplicates from the exception reporter and the users describe symptoms 
as a loop of exception after opening any PHP file

The risk is very low - I added a trivial check for NPE that was missing 
before (although it was not failing until very recently)

The change is http://hg.netbeans.org/release67/rev/5cca583d401c reviewed 
by R. Matous. The QE still cannot reproduce the problem, but I got a 
green light for integration from Marian Mironovic.