Ensure decompressed metadata is null-terminated

[neilmayhew]

Fixes #35, Refresh hangs

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This change is

[neilmayhew]

I realized that g_strndup was a cleaner and more efficient way to do it.

[ximion]

I made some more general improvements on that code now, it should be much easier to read and also be a bit more friendly to memory now (the previous code was in some occasions duplicating the data twice, which didn't make any sense).