Dear all,

the attached obvious patch fixes an ICE on a NULL pointer
dereference.  We didn't properly check that the types of
expressions are character before referencing the length.

The issue was originally investigated by Steve, so I made
him co-author.

Regtested on x86_64-pc-linux-gnu and pushed to mainline as
commit r13-2690-g7bd4deb2a7c1394550610ab27507d1ed2af817c2

Thanks,
Harald