loader: Fix read_targphys() to behave when read() fails

Happily passes (size_t)-1 to rom_add_blob_fixed(), which promptly dies
attempting to malloc that much.  Spotted by Coverity.

Bonus fix for ROMs larger than INT_MAX bytes: return ssize_t instead
of int.  Bug can't bite, because the only user load_aout() limits ROM
size to an int value.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
