Fixed possible dereferencing of nullptr.
Fixed possible dereferencing of nullptr.

clang++ --analyze warnings:

newbase/NFmiLevelBag.cpp:73:5: warning: Forming reference to null pointer
    itsLevels[i] = theLevelArray[i];
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
newbase/NFmiLevelBag.cpp:90:5: warning: Called C++ object pointer is null
    itsLevels[i] = (theBag.itsLevels)[i];
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

itsLevels may be initialized to nullptr. Added an if-clause around the loop.

fmidev/smartmet-library-newbase,fmidev/smartmet-library-newbase