sentinel: check nil PGState.
sentinel: check nil PGState.

Before reading the values in PGState, GetBestStandby should check if PGState is
nil. This can happen if a new member appears but its PGState isn't available
(because the db is not yet completely started).

siepkes/stolon,sgotti/stolon,sorintlab/stolon,siepkes/stolon,sgotti/stolon,sorintlab/stolon,smkingsoft/stolon,smkingsoft/stolon