code-style: simplify some checks

code-style: simplify some checks

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.