Bug 3344: Make sure StackedPathArgument stack is non empty.

Bug 3344: Make sure StackedPathArgument stack is non empty.

The while cycle in StackedYangInstanceIdentifier did double
check of tryPathArguments for same instance identifier,
which led to shared state during creation. This may have
resulted in StackedPathArgumements with empty stack.

Changing it into do-while cycle makes sure stack is non empty
and also state during construction is not shared between
multiple invokers.

Change-Id: I834c1f22c477bb03a6bae9c4a366308a0988ce4e
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>