tiny_ssim: mv read error checks closer to assignment

Quiets some spurious -Wmaybe-uninitialized warnings with gcc 14.1.0.

In function 'calc_plane_error16',
    inlined from 'main' at ../tools/tiny_ssim.c:464:5:
../tools/tiny_ssim.c:37:12: warning: 'v[0]' may be used uninitialized
  [-Wmaybe-uninitialized]
   37 |   if (orig == NULL || recon == NULL) {
      |            ^
In function 'calc_plane_error16',
    inlined from 'main' at ../tools/tiny_ssim.c:462:5:
../tools/tiny_ssim.c:37:12: warning: 'u[0]' may be used uninitialized
  [-Wmaybe-uninitialized]
   37 |   if (orig == NULL || recon == NULL) {
      |            ^
In function 'calc_plane_error',
    inlined from 'main' at ../tools/tiny_ssim.c:461:5:
../tools/tiny_ssim.c:61:12: warning: 'y[0]' may be used uninitialized
  [-Wmaybe-uninitialized]
   61 |   if (orig == NULL || recon == NULL) {

To reduce confusion, read_input_file() is changed to return an int as
previously it would only return (size_t)-1/0/1 (and now returns 0/1).

Change-Id: I2344048ecc2bd233891ffcef08002ee98d6d262a
1 file changed