retract_all/1 crashes if retracting from undefined module (#906)

retract_all/1 causes a segmentation violation if called in the context of a 
undefined module:


Version 5.1.1, Thu Feb 22 01:08 2001
[eclipse 1]: retract_all(a(_))@notthere.
Segmentation violation - possible reasons are:
- a faulty external C function
- certain operations on circular terms