Initialize the dumper struct before calling set_dumper().

Fields owned by the generic code were being left uninitialized,
causing problems in clear_dumper() if an error occurred.

Coverity CID:	1391200
X-MFC with:	r333283

sys/netinet/netdump/netdump_client.c

@@ -1204,6 +1204,7 @@ netdump_ioctl(struct cdev *dev __unused, u_long cmd, caddr_t addr,
 			}
 		}
 
+		memset(&dumper, 0, sizeof(dumper));
 		dumper.dumper_start = netdump_start;
 		dumper.dumper_hdr = netdump_write_headers;
 		dumper.dumper = netdump_dumper;