Handle error when we don't have Eagle Board

This can also occur in libraries, so ensure we're not dereferencing a
null board pointer

Fixes https://gitlab.com/kicad/code/kicad/-/issues/18702

(cherry picked from commit 89b9b13e21)

pcbnew/pcb_io/eagle/pcb_io_eagle.cpp

@@ -2035,7 +2035,12 @@ void PCB_IO_EAGLE::packagePad( FOOTPRINT* aFootprint, wxXmlNode* aTree )
     }
     else
     {
-        wxLogError( _( "Invalid zero-sized pad ignored in\nfile: %s" ), m_board->GetFileName() );
+        wxFileName fileName( m_lib_path );
+
+        if( m_board)
+            wxLogError( _( "Invalid zero-sized pad ignored in\nfile: %s" ), m_board->GetFileName() );
+        else
+            wxLogError( _( "Invalid zero-sized pad ignored in\nfile: %s" ), fileName.GetFullName() );
     }
 }