astr:fix byte length calculation in remove method (from Zile).

* zee/astr.lua (remove): When removing a chunk, we shift down the
bytes between the end of the removed chunk and the end of the
buffer; most definitely not the same number of bytes we happen to
be overwriting!

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
Merge pull request #7 from gvvaughan/patch-4

astr:fix byte length calculation in remove method (from Zile).