bus error /segmentation fault from 'approx' (PR#7177)

Full_Name: joerg  van den hoff
Version: 1.9.1
OS: MacOS  and SunOS 
Submission from: (NULL) (149.220.4.88)


follow up to ID 7166. something like

approx(c(1,2),c(NA,NA),1.5,rule=2)

crashes 1.9.1 on both systems (MacOS 10.3.5.: bus error,  SunOS 5.9:
segmentation fault) even if xout is within given x range (as in example above)
where rule=2 seems not be relevant anyway.