Omit a few minor unnecessary range checks

Omit a few minor unnecessary range checks

Based on Pip Cet’s review (Bug#36370#19).
* src/fileio.c (Fdo_auto_save):
* src/image.c (lookup_image):
* src/textprop.c (Fnext_single_char_property_change):
Prefer XFIXNUM to XFIXNAT for clarity and consistency with
neighboring code, and to avoid unnecessary range checks.
* src/image.c (lookup_image): Omit unnecessary range checks.