qemu-img: fix offset calculation in bench

qemu-img: fix offset calculation in bench

This error was discovered by fuzzing qemu-img.

The current offset calculation leads to an EIO error
in block/block-backend.c: blk_check_byte_request():

 if (offset > len || len - offset < bytes) {
     return -EIO;
 }

This triggers the error message:
"qemu-img: Failed request: Input/output error".

Example of the issue:
 offset: 260076
 len: 260096
 bytes: 4096

This fix ensures that offset remains within a valid range.

Signed-off-by: Denis Rastyogin <gerben@altlinux.org>
Message-ID: <20250506141410.100119-1-gerben@altlinux.org>
[kwolf: Fixed up integer overflow]
Signed-off-by: Kevin Wolf <kwolf@redhat.com>