Fix segmentation fault in func_user isPure
Fix segmentation fault in func_user isPure

The isPure function of func_user nodes called isPure recursively on
both subnodes instead of just one. Combined with missing nullpointer
checks in this function a call to isPure on any expression
containg an user provided function would crash.

VirtualRaven/mathlibra,VirtualRaven/mathlibra,VirtualRaven/mathlibra,VirtualRaven/mathlibra