SwitchManager: getSwitchState() should not return null

SwitchManager: getSwitchState() should not return null

getSwitchState method populates switch state to switches in thread-safe
way. When race occurred, it should return already populated one.
The callers of it, updateSwitch and updateSwitchConfig, access the result
without null check. This is found by code review.
This semantics is same to other similar helper methods, getEPNGSet(),
getConfdGroupOrdinal(), getContextOrdinal().

Change-Id: I88d03a8f0b0109921377bc1fd4628a072c6b0552
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>