MuonRegionSelector: Fix gcc 11 compilation warning.

Check the result of dynamic_cast before dereferencing.

MuonSpectrometer/MuonDetDescr/MuonRegionSelector/src/MuonRegSelCondAlg.cxx

@@ -77,7 +77,11 @@ StatusCode MuonRegSelCondAlg::execute(const EventContext& ctx )  const
 
   // write out new new LUT to a file if need be
   
-  if ( m_printTable ) dynamic_cast<const RegSelSiLUT*>(rd.get())->write( name()+".map" );
+  if ( m_printTable ) {
+    if (auto lut = dynamic_cast<const RegSelSiLUT*>(rd.get())) {
+      lut->write( name()+".map" );
+    }
+  }
 
   /// create the conditions data for storage