basic/fileio: simplify calculation of buffer size in read_full_virtual_file()

basic/fileio: simplify calculation of buffer size in read_full_virtual_file()

We'd first assign a value up to SSIZE_MAX, and then immediately check if we
have a value bigger than READ_FULL_BYTES_MAX. This wasn't exactly wrong, but a
bit roundabout. Let's immediately assign the value from the appropriate range
or error out.

Coverity CID#1450973.