lib: Fix fpi_img_is_sane()

The checks weren't:
- checking whether the width or height were negative
- whether img->width * img->height would overflow, or
  was bigger than G_MAXINT
- whether img->width * img->height was bigger than the total
  length of the buffer

The last one looks like a thinko, it checked for:
(img->length * img->height) < img->length
which is equivalent to:
img->height < 1
which we already check for earlier.

Closes: #85
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00:02:30