I used the construct like f_rnd_octstring(f_rnd_int(100)) in a number
of places to generate random-length packets with randomized length.
The problem I didn't realize is that f_rnd_int() of course can also
return '0', which would generate zero-length packets. This may be
permitted in some protocols, but it leads to problems e.g. when trying
to send a UDP packet of zero length (which the kernel will not do).
So let's introduce
* f_rnd_int_nonzero() for returning non-zero randomized integers
* f_rnd_octstring_rnd_len() for returning a random-length random payload
* replace all f_rnd_octstring(f_rnd_int()) call sites with the new