Handle errors from run_ofctl() when dumping flows

The function dump_flows_for_table() calls run_ofctl(). If this occurs during an OVS
restart, run_ofctl() will return None. dump_flows_for_table() needs to realize this
and not try to call splitlines() on a None object.

Change-Id: Ide26028656e634113f955dfc89569149d4d5b306
Closes-Bug: #1324659
Merge "Handle errors from run_ofctl() when dumping flows"