input: reassemble_frags(): don't assert() on protobuf unpack error
input: nmsg_input_read_null(): convert assertion into an nmsg_res_parse_error return
input: _input_nmsg_unpack_container(): convert assertions on protobuf unpack failures into nmsg_res_parse_error returns
input: _input_nmsg_read_container_sock: convert assertion to nmsg_res_parse_error return