metalink: fix improbable crash parsing metalink filename

The this_url pointer wasn't being initialized, so if strdup() would return
null when copying the filename in a metalink file, then hilarity would
ensue during the cleanup phase. This change was brought to you by clang,
which noticed this and raised a warning.
FEATURES: why yes, we do support metalink

I just noticed Metalink support wasn't listed as a feature of the tool.