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>