media: v4l2-tpg: prevent the risk of a division by zero

commit e6a3ea83fbe15d4818d01804e904cbb0e64e543b upstream.

As reported by Coverity, the logic at tpg_precalculate_line()
blindly rescales the buffer even when scaled_witdh is equal to
zero. If this ever happens, this will cause a division by zero.

Instead, add a WARN_ON_ONCE() to trigger such cases and return
without doing any precalculation.

Fixes: 63881df9 ("[media] vivid: add the Test Pattern Generator")
Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>