validate rowBytes and width overflow in avifImageRGBToYUV

[uwezkhan]

Added validation in avifImageRGBToYUV to make sure rowBytes and width are safe before processing.

What was the issue

The function was using rowBytes directly without checking if it was actually large enough for the given width and pixel size.
If rowBytes is smaller than expected, it can read past the buffer.
Also, width * pixelSize could overflow in some cases.

What I changed

• Added a check to prevent overflow in width * pixelSize
• Added a check to ensure rowBytes is not smaller than required

Impact

This prevents out-of-bounds reads when invalid RGB input is passed.