main change here is making the `zoom()` logic saner. the rest of the
changes are just small stuff which accumulated on my local branch.
pop() must not be called with NULL. and `zoom()` achieves this, but in a
very (unnecessarily) complicated way:
if c == NULL then nexttiled() will return NULL as well, so we enter this
branch:
if (c == nexttiled(selmon->clients))
in here the !c check fails and the function returns before calling pop()
if (!c || !(c = nexttiled(c->next)))
return;
however, none of this was needed. we can simply return early if c was NULL.
Also `c` is set to `selmon->sel` so we can use `c` in the first check
instead which makes things shorter.
maybe leak isn't the best word, given that the object lives for the
entire duration of the program's lifetime.
however, all elements of scheme are free-ed, can't think of any reason
why scheme itself should be an exception.
