eeschema: guard netclass lookup during schematic parse

Malformed schematic content can reference a missing netclass and crash
parse-time item setup. Check lookup results before use and fall back
safely when the netclass is absent.

Found by codex-5.3

eeschema/sch_item.cpp

@@ -512,16 +512,21 @@ std::shared_ptr<NETCLASS> SCH_ITEM::GetEffectiveNetClass( const SCH_SHEET_PATH*
 
     SCHEMATIC* schematic = Schematic();
 
-    if( schematic )
-    {
-        std::shared_ptr<NET_SETTINGS>& netSettings = schematic->Project().GetProjectFile().m_NetSettings;
-        SCH_CONNECTION* connection = Connection( aSheet );
+    if( !schematic || !schematic->IsValid() )
+        return nullNetclass;
 
-        if( connection )
-            return netSettings->GetEffectiveNetClass( connection->Name() );
-        else
-            return netSettings->GetDefaultNetclass();
-    }
+    std::shared_ptr<NET_SETTINGS>& netSettings = schematic->Project().GetProjectFile().m_NetSettings;
+
+    if( !netSettings )
+        return nullNetclass;
+
+    SCH_CONNECTION* connection = Connection( aSheet );
+
+    if( connection )
+        return netSettings->GetEffectiveNetClass( connection->Name() );
+
+    if( std::shared_ptr<NETCLASS> defaultNetclass = netSettings->GetDefaultNetclass() )
+        return defaultNetclass;
 
     return nullNetclass;
 }