Send the hosts count to the client as -1, because the invalid target list

[jjnicola]

What:
Send the hosts count to the client as -1, because the invalid target list

Why:
This allows ospd to finish the scan gracefully, setting the scan to finished
instead of interrupted, but sending the error message as well.

How:

• Set debug log level in ospd.
• Start a scan with an invalid target list (only possible via gvm-cli, as gsa wont you to create a invalid target).
• See in the log that ospd-openvas receives the update of amount of host from openvas set to -1

Checklist:

• Tests
• CHANGELOG Entry