update-flow RPC causes NPE if either match or priority in UpdateFlowInput is null.

OFRpcTaskFactory.createUpdateFlowTask():

boolean updatedFlow = (getInput().getUpdatedFlow().getMatch().equals(getInput().getOriginalFlow().getMatch())) &&
(getInput().getUpdatedFlow().getPriority().equals(getInput().getOriginalFlow().getPriority()));

In addition, update-flow RPC should remove original flow and add updated flow
if any of the following fields is changed because FLOW_MOD (MODIFY/MODIFY_STRICT)
will not change them.

• idle-timeout
• hard-timeout
• flags
• cookie

OF13 FLOW_MOD (MODIFY) can be filtered by cookie and cookie_mask.
So one FLOW_MOD (MODIFY) should be sent if all of the following conditions are met:

• Protocol version is OF13.
• Strict flag is not set in UpdatedFlow.
• A non-zero cookie_mask is set in UpdatedFlow.
• Flow match, priority, idle-timeout, hard-timeout, and flags are not changed.