#4611 Lisp errors from cf

[Raymond Toy]

As mentioned in the mailing list thread,

We should handle this better.

For example:
```
(%i2) (3/17)*[1, -2, 5]/sqrt(11) + (8/13);
3 8 8 6 15 8
(%o2) [─────────── + ──, ── - ───────────, ─────────── + ──]
17 sqrt(11) 13 13 17 sqrt(11) 17 sqrt(11) 13
(%i3) cf(%);

Maxima encountered a Lisp error:

The value
((MPLUS SIMP) ((RAT SIMP) 8 13)
((MTIMES SIMP) ((RAT SIMP) 15 17) ((MEXPT SIMP) 11 ((RAT SIMP) -1 2))))

is not of type
NUMBER
```

We should handle this better; the user shouldn't get a Lisp error.