Fix copy-pasta

testing 'othervia' for dynamic_cast success needs to use `othervia`
rather than `via` which might still be null

Fixes Sentry issue KICAD-5K

pcbnew/drc/drc_test_provider_solder_mask.cpp

@@ -499,7 +499,7 @@ void DRC_TEST_PROVIDER_SOLDER_MASK::testItemAgainstItems( BOARD_ITEM* aItem, con
 
                 if( otherPad )
                     clearance += otherPad->GetSolderMaskExpansion();
-                else if( otherVia && !via->IsTented() )
+                else if( otherVia && !otherVia->IsTented() )
                     clearance += otherVia->GetSolderMaskExpansion();
 
                 if( itemShape->Collide( otherShape.get(), clearance, &actual, &pos ) )