Fix potential division by zero in shaper_reset()

Fix potential division by zero in shaper_reset()

shaper_reset() is only ever called with "bytes_per_second" set to
a non-zero value - so the whole check "is it zero? if not, use
constrain_int() to make sure it is within bounds" is not needed ->
reduce check to just constrain_int() so even if somebody would
call shaper_reset(..., 0) it would not lead to a div-by-zero.

Found by Coverity.

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <1478598242-23514-1-git-send-email-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg12942.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>