scanarg(s, del) never returns s; the empty field results in s + 1.
Restore the correct checks, and move NUL-termination into scanarg(),
while we are at it.

Incidentally, mixing "coding style cleanups" (for small values of cleanup)
with functional changes is a Bad Idea(tm)...
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>