intbitset: no crash when intbitset is on rhs

* When an intbitset is on the right hand side of a foreign type
  operator (e.g. a set), do no longer crash with a segmentation
  fault, rather raises a TypeError.  (closes #1287)

Reported-by: Thorsten Schwander <thorsten.schwander@gmail.com>
Tested-by: Tibor Simko <tibor.simko@cern.ch>