[snmp]fix set system parameter

modules/Ethernet/snmp_api.c

@@ -226,7 +226,7 @@ void SNMP_SetWriteCommunity(char *comm)
   */
 void SNMP_SetSysContact(SNMP_SYS_CONTACT_t *con)
 {
-  snmp_mib2_set_syscontact((u8_t*)con->contact, &con->len, sizeof(con->contact));
+  snmp_mib2_set_syscontact((u8_t*)con->contact, &con->len, (sizeof(con->contact)-1));
 }
 
 /**
@@ -235,7 +235,7 @@ void SNMP_SetSysContact(SNMP_SYS_CONTACT_t *con)
   */
 void SNMP_SetSysName(SNMP_SYS_NAME_t *name)
 {
-  snmp_mib2_set_sysname((u8_t*)name->name, &name->len, sizeof(name->name));
+  snmp_mib2_set_sysname((u8_t*)name->name, &name->len, (sizeof(name->name)-1));
 }
 
 /**
@@ -244,7 +244,7 @@ void SNMP_SetSysName(SNMP_SYS_NAME_t *name)
   */
 void SNMP_SetSysLocation(SNMP_SYS_LOCATION_t *loc)
 {
-  snmp_mib2_set_syslocation((u8_t*)loc->location, &loc->len, sizeof(loc->location));
+  snmp_mib2_set_syslocation((u8_t*)loc->location, &loc->len, (sizeof(loc->location)-1));
 }
 
 /**