Fix assert from IsPower in search_handlers.cpp

Fixes KICAD-2C6

eeschema/widgets/search_handlers.cpp

@@ -149,7 +149,8 @@ int SYMBOL_SEARCH_HANDLER::Search( const wxString& aQuery )
                 {
                     SCH_SYMBOL* sym = dynamic_cast<SCH_SYMBOL*>( item );
 
-                    if( sym->IsPower() )
+                    // IsPower depends on non-missing lib symbol association
+                    if( !sym->IsMissingLibSymbol() && sym->IsPower() )
                         return false;
 
                     for( SCH_FIELD& field : sym->GetFields() )