fix(battery): Safety check

Make sure returned label is valid before access.

Refs #318

src/modules/battery.cpp

@@ -198,12 +198,14 @@ namespace modules {
       }
     }();
 
+    if (label) {
       label->reset_tokens();
       label->replace_token("%percentage%", to_string(m_percentage) + "%");
 
       if (m_state != battery_module::state::FULL && !m_timeformat.empty()) {
         label->replace_token("%time%", current_time());
       }
+    }
 
     return true;
   }