You cannot use busybox nc with Option -z, because -z is mapped to -o, 
e.g. I used the following command, to test if there is a running ssh 
service on somehostname:

busybox nc -w 2 -z somehostname 22 ; echo $?

This fails with a segfaulting busybox. I tracked the error and found
it was introduced with the commit de6f14800675cd0401106876da81da7618de71c6
(nc_bloaty: support -ll and -lk). 

Please apply the following patch to correct this error:

Index: busybox-1.22.1/networking/nc_bloaty.c
===================================================================
--- busybox-1.22.1.orig/networking/nc_bloaty.c
+++ busybox-1.22.1/networking/nc_bloaty.c
@@ -175,9 +175,9 @@ enum {
 	OPT_w = (1 << 5),
 	OPT_l = (1 << 6) * ENABLE_NC_SERVER,
 	OPT_k = (1 << 7) * ENABLE_NC_SERVER,
-	OPT_i = (1 << (7+2*ENABLE_NC_SERVER)) * ENABLE_NC_EXTRA,
-	OPT_o = (1 << (8+2*ENABLE_NC_SERVER)) * ENABLE_NC_EXTRA,
-	OPT_z = (1 << (9+2*ENABLE_NC_SERVER)) * ENABLE_NC_EXTRA,
+	OPT_i = (1 << (6+2*ENABLE_NC_SERVER)) * ENABLE_NC_EXTRA,
+	OPT_o = (1 << (7+2*ENABLE_NC_SERVER)) * ENABLE_NC_EXTRA,
+	OPT_z = (1 << (8+2*ENABLE_NC_SERVER)) * ENABLE_NC_EXTRA,
 };
 
 #define o_nflag   (option_mask32 & OPT_n)