evdev: avoid bit vector decoding on non-successful and 0 return codes

evdev: avoid bit vector decoding on non-successful and 0 return codes

Reported by Clang.

    strace/evdev.c:157:3: note: The value 0 is assigned to 'size'
    #                size = tcp->u_rval * 8;
    #                ^~~~~~~~~~~~~~~~~~~~~~
    strace/evdev.c:158:2: warning: Declared variable-length array (VLA)
    has zero size
    #        char decoded_arg[size];
    #        ^

* evdev.c (decode_bitset_): Bail out before decoded_arg VLA definition.