Fix issue when resizing images in RTE

What does this MR do and why?

When resizing images, we get dimensions from the DOM to begin resize. But if the dimensions are "auto", doing a parseInt on them results in NaN. In that case we should fall back to computed values of width and height.

MR against master: !163382 (merged)

It was a regression from !161676 (merged) which was released in 17.3.

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