Throw away tmp files if mv fails

Attempt to get rid of stale temp files that occasionally drop. While
there is anecdotal evidence that this doesn't solve the problem, it
doesn't hurt, and it's the only place I can see to fix it.

z.sh

@@ -80,7 +80,7 @@ _z() {
   if [ $? -ne 0 -a -f "$datafile" ]; then
    env rm -f "$tempfile"
   else
-   env mv -f "$tempfile" "$datafile"
+   env mv -f "$tempfile" "$datafile" || env rm -f "$tmpfile"
   fi
 
  # tab completion