#5278 killcontext doesn't always refuse to kill an active context

[David Scherfgen]

Detected by Claude:

killcontext should never kill an active context. However, in this example it does:

supcontext(a)$
assume(p > 0)$
supcontext(b)$
is(p > 0);      /* true */
killcontext(a); /* done - it killed it, even though it's active */
is(p > 0);      /* unknown */
contexts;       /* [b, initial, global] */

If the first is(p > 0) is removed, killcontext correctly refuses.