Steps to reproduce:

1. Create an item with Numeric unsigned type and preprocessing with simple change type and Zabbix trapper item type.
2. Reload Zabbix configuration
3. Execute the next Zabbix sender values

   bash-4.3# zabbix_sender -z 127.0.0.1 -s test -k test_trap -o 20
   info from server: "processed: 1; failed: 0; total: 1; seconds spent: 0.000171"
   sent: 1; skipped: 0; total: 1
   bash-4.3# zabbix_sender -z 127.0.0.1 -s test -k test_trap -o 100
   info from server: "processed: 1; failed: 0; total: 1; seconds spent: 0.000141"
   sent: 1; skipped: 0; total: 1
   bash-4.3# zabbix_sender -z 127.0.0.1 -s test -k test_trap -o 100
   info from server: "processed: 1; failed: 0; total: 1; seconds spent: 0.000078"
   sent: 1; skipped: 0; total: 1
   bash-4.3# zabbix_sender -z 127.0.0.1 -s test -k test_trap -o -1
   info from server: "processed: 1; failed: 0; total: 1; seconds spent: 0.000174"
   sent: 1; skipped: 0; total: 1
   bash-4.3# zabbix_sender -z 127.0.0.1 -s test -k test_trap -o 100
   info from server: "processed: 1; failed: 0; total: 1; seconds spent: 0.000099"
   sent: 1; skipped: 0; total: 1
   

Result:
-1 value does not cause "not supported" state and then Zabbix calculates (curr_val - prev_val), where prev_val = -1, so Zabbix calculates curr_val + 1.
Expected:
-1 value should cause "not supported" state or must be ignored.