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

In the Linux kernel, the following vulnerability has been resolved:

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

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.