del_zeros() may convert invalid number into a valid number

Steps to reproduce:

1. Create an item, type "Zabbix trapper", information type "numeric (float)"
2. ./zabbix_sender -z 127.0.0.1 -s "host" -k "float.trapper" -o "1.2345.000"
3. Add preprocessing step "Custom multiplier", multiply by 1
4. ./zabbix_sender -z 127.0.0.1 -s "host" -k "float.trapper" -o "1.2345.000"

Result:

Step 2 fails as expceted.

Step 4 succeeds, value "1.23" is reported in Monitoring -> Latest data. Opening Graph and switching view from Graph to Values shows that stored value is 1.2345.

Expected:

Both ./zabbix_sender calls should fail.