STORM-3051:some Potential NPEs

[lujiefsi]

We have developed a static analysis tool NPEDetector to find some potential NPE. Our analysis shows that some callees may return null in corner case(e.g. node crash , IO exception), some of their callers have !=null check but some do not have.

Bug:

1. callee CgroupCenter#getSubSystems return null when meet exception:

} catch (Exception e) {
LOG.error("Get subSystems error {}", e);
}
return null;

but its caller use it without check:

public boolean isSubSystemEnabled(SubSystemType subSystemType) {
  Set subSystems = this.getSubSystems();
  for (SubSystem subSystem : subSystems) {
     if (subSystem.getType() == subSystemType) {
     return true;
  }
  }
  return false;
}

other callee and caller pair that have same problem.

2. callee RAS_Node#getUsedSlots and caller RAS_Node#totalSlotsUsed
3. CgroupCenter#getHierarchies and caller CgroupCenter#isMounted,

[HeartSaVioR]

nit: space between ) and {