Fix reverb buffer-overrun issue

Add code to check the upper limit of the index.  TiMidity++ to buffer-
overrun when the value of the reverb_status_gs.character was a 6 or 7.
info->index is set to a value out of range when the value of the
"Reverb Time" was -64.

Signed-off-by: yta <yta@inter7.jp>