Only initialize a reactor if it exists

Windows users reported a crash on starting a level with no reactor.
derhass traced it to an unconditional initialization of
Objects[cntrlcen_objnum] members, even if cntrlcen_objnum is -1.  Fix
this by checking for the reactor before accessing it.

main/cntrlcen.c

@@ -456,7 +456,7 @@ void init_controlcen_for_level(void)
 			Objects[cntrlcen_objnum].render_type = RT_NONE;
 			Control_center_present = 0;
 		}
-	} else {
+	} else if (cntrlcen_objnum != -1) {
 		//	Compute all gun positions.
 		objp = &Objects[cntrlcen_objnum];
 		reactor *reactor = get_reactor_definition(objp->id);