Fix a number of issues identified by FindBugs

...net/floodlightcontroller/core/OFMessageFilterManager.java

...va/net/floodlightcontroller/core/internal/Controller.java

...a/net/floodlightcontroller/core/internal/RoleChanger.java

...lightcontroller/core/web/AllSwitchStatisticsResource.java

...tcontroller/devicemanager/web/AbstractDeviceResource.java

...main/java/net/floodlightcontroller/firewall/Firewall.java

...n/java/net/floodlightcontroller/flowcache/FCQueryObj.java

...a/net/floodlightcontroller/loadbalancer/LoadBalancer.java

src/main/java/net/floodlightcontroller/packet/BSNPROBE.java

.../java/net/floodlightcontroller/packet/DHCPPacketType.java

src/main/java/net/floodlightcontroller/packet/Ethernet.java

...et/floodlightcontroller/packet/LLDPOrganizationalTLV.java

src/main/java/net/floodlightcontroller/packet/TCP.java

...in/java/net/floodlightcontroller/storage/RowOrdering.java

src/main/java/net/floodlightcontroller/util/LoadMonitor.java

...in/java/org/openflow/protocol/vendor/OFBasicVendorId.java

src/main/java/org/openflow/util/HexString.java

.../floodlightcontroller/core/internal/OFSwitchImplTest.java

...net/floodlightcontroller/core/util/SingletonTaskTest.java

...troller/devicemanager/internal/DeviceManagerImplTest.java