math/limlee.c: Handle an abort from `pgen' correctly.

The `llgen' function just tries again if `pgen' reports an abort.  This
is entirely contrary to the intend of the `PGEN_ABORT' protocol, so I've
no idea why I thought this was a good idea.

Instead, leave the prime slot null (because adding a return code to the
`pgen' callback breaks the API), and arrange for the caller to notice
and clean up.  This is annoyingly because there may be an `mpmul' in
progress.