staging: sb105x: fix potential NULL pointer dereference in mp_chars_in_buffer()

The dereference to 'state' should be moved below the NULL test.

Cc: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>