Defensive programming measures.

Recent issue conversations are suggesting that it's no possible for
footprints to contain null fields.  Rather than blindly dereferencing
field pointers in loops, check to make sure the field pointer is not
null before dereferencing.

pcbnew/plot_brditems_plotter.cpp

@@ -433,6 +433,8 @@ void BRDITEMS_PLOTTER::PlotFootprintTextItems( const FOOTPRINT* aFootprint )
     // Skip the reference and value texts that are handled specially
     for( PCB_FIELD* field : aFootprint->GetFields() )
     {
+        wxCHECK2( field, continue );
+
         if( field->IsReference() || field->IsValue() )
             continue;