term: Fix overflow on user inputs
    
    This requires a very weird input from the serial interface but can cause
    an overflow in input_buf (keys) overwriting the next variable (npending)
    with the user choice:
    
    (pahole output)
    
    struct grub_terminfo_input_state {
            int                        input_buf[6];         /*     0    24 */
            int                        npending;             /*    24     4 */ <- CORRUPT
            ...snip...
    
    The magic string requires causing this is "ESC,O,],0,1,2,q" and we overflow
    npending with "q" (aka increase npending to 161). The simplest fix is to
    just to disallow overwrites input_buf, which exactly what this patch does.
    
    Fixes: CID 292449
    
    Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
    Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>