Fix crash in QgsVectorFileWriter::addFeature()

createFeature() might return a NULL pointer that OGR_L_CreateFeature()
will crash upon when GDAL is compiled in debug mode (where null
pointer assertions are turned on)
Merge pull request #1460 from rouault/fix_crash_in_QgsVectorFileWriter_addFeature

Fix crash in QgsVectorFileWriter::addFeature()