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
1 job from wip/hadess/small-bugs in 2 minutes and 30 seconds (queued for 2 seconds)
Status Job ID Name Coverage
passed #28537