Bug 12721 - Prevent software error if incorrect fieldnames given in sypref Statistics...

Bug 12721 - Prevent software error if incorrect fieldnames given in sypref StatisticsFields

To reproduce issue:
See comment #1

To test:
- Apply patch
- Leave syspref StatisticsFields empty
- Display statistics for an author

  => Result: Table displays Shelving location, Collection code, Item type
           (as before)

- Change syspref to any combination of location|itype|ccode
  => Result: Table displays columns as appropriate

- Change syspref to some garbage
  => Result: Same as with empty syspref (was crashing without patch)

- Change syspref to valid combination with trailing |
  => Result: Table displays columns as appropriate (was crashing
     without patch)

- Change syspref to a combination of valid and invalid fields
  (location|blah|ccode)
  => Result: Table displays column of valid fields only (was crashing
     without patch)

Signed-off-by: Aleisha <aleishaamohia@hotmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>