cells: fix IAE in 'show pinboard'

we get IllegalArgumentException if number of entries in the pinboard
less that we have to skip:

java.lang.IllegalArgumentException: number to skip cannot be negative
	at com.google.common.base.Preconditions.checkArgument(Preconditions.java:125) ~[guava-17.0.jar:na]
	at com.google.common.collect.Iterables.skip(Iterables.java:846) ~[guava-17.0.jar:na]
	at dmg.util.Pinboard.dump(Pinboard.java:44) ~[cells-2.11.0-SNAPSHOT.jar:2.11.0-SNAPSHOT]
	at dmg.cells.nucleus.CellAdapter$ShowPinboardCommand.call(CellAdapter.java:676) ~[cells-2.11.0-SNAPSHOT.jar:2.11.0-SNAPSHOT]
	at dmg.cells.nucleus.CellAdapter$ShowPinboardCommand.call(CellAdapter.java:657) ~[cells-2.11.0-SNAPSHOT.jar:2.11.0-SNAPSHOT]
	at org.dcache.util.cli.AnnotatedCommandExecutor.execute(AnnotatedCommandExecutor.java:128) ~[common-cli-2.11.0-SNAPSHOT.jar:2.11.0-SNAPSHOT]
	at org.dcache.util.cli.CommandInterpreter$CommandEntry.execute(CommandInterpreter.java:260) ~[common-cli-2.11.0-SNAPSHOT.jar:2.11.0-SN

Acked-by: Gerd Behrmann
Target: master, 2.10
Require-book: no
Require-notes: no