Fix maybe-unitialized warning observed in AtlasExternals+Athnea in RelWithDebInfo mode

source/geometry/magneticfield/src/G4MagIntegratorDriver.cc

@@ -115,7 +115,10 @@ G4MagInt_Driver::AccurateAdvance(G4FieldTrack& y_current,
 #endif
 
   G4double y[G4FieldTrack::ncompSVEC], dydx[G4FieldTrack::ncompSVEC];
-  G4double ystart[G4FieldTrack::ncompSVEC], yEnd[G4FieldTrack::ncompSVEC]; 
+  G4double ystart[G4FieldTrack::ncompSVEC];
+  G4double yEnd[G4FieldTrack::ncompSVEC] = {0., 0., 0., 0.,
+	                                    0., 0., 0., 0.,
+		                            0., 0., 0., 0.};
   G4double  x1, x2;
   G4bool succeeded = true, lastStepSucceeded;