cmd/gc: empty slices and strings on stack can cause memory leak

<pre>This is only a mental experiment.
Empty strings and slices that point after the last element (e.g s2 := s1[len(s1):]) can
produce a pointer to the next memory block. If scanned w/o rich type info, such pointers
will pin the next memory block.
Such scanning happens for stacks (and some runtime frames, e.g. concatstring).
Probably we want set such pointers to nil.</pre>
