fs: Prevent overflows when assigning returned values from read_number()

fs: Prevent overflows when assigning returned values from read_number()

The direct assignment of the unsigned long long value returned by
read_number() can potentially lead to an overflow on a 32-bit systems.
The fix replaces the direct assignments with calls to grub_cast()
which detects the overflows and safely assigns the values if no
overflow is detected.

Signed-off-by: Lidong Chen <lidong.chen@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>