asn_internal: Call va_end before returning

[lnksz]

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I do not know the project's code-style, in some C projects this kind of error-handling would incorporate a goto. But because of the general avoidance of goto I added va_end before each return.

I suppose has more insight into what this function does, one could "dry" the error handling.

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Coverage decreased (-0.03%) to 70.883% when pulling 834b75aeb4596ca558130acc8ace21b3f3a7cdac on lnksz:patch-1 into 00fa516f3c6ab247c263df6828143c34b1fbffa4 on vlm:master.