Hello,

This is an interesting bug, because the only way to hit it (as far as I can
tell) is to mess up when writing a compiler. However, I did mess up, and I
discover that I can generate a `call' instruction in the trunk VM where the
procedure to call will be 0x0. Then the VM will try to check whether the
procedure is really a procedure, and Guile will segfault at line 796 of
v-i-system.c.

I think the correct behavior would be to throw a `vm-bad-instruction' error
instead. The fix should be pretty simple - just check if program is 0x0 and
jump to vm-bad-instruction in that case.

Noah