input: nmsg_input_read_null(): fix inverted filter logic
input: nmsg_input_read_null(): fix inverted filter logic

_input_nmsg_filter returns true if the payload is allowed by the filter and
false if it is not allowed. However, the call to _input_nmsg_filter in
nmsg_input_read_null was assuming the opposite semantics.

[edmonds@fsi.io: adjust commit message.]

hstern/nmsg,hstern/nmsg,hstern/nmsg,hstern/nmsg,hstern/nmsg