Fix response memory handling in handle_osp_scan

[timopollmeier]

What:
When getting only the progress, the XML string is no longer requested
and when getting the full report, the string is freed in case of an
error.

Why:
The XML from the progress requests was never freed until the end of the scan,
so the strings kept consuming more and more memory, which could cause
problems during longer scans.

How did you test it:
By observing the memory consumption of the scan handler process in htop and
profiling gvmd with the massif tool of Valgrind while scanning a large range of
alive hosts.

Checklist:

• Tests
• CHANGELOG Entry