This commit addresses two issues:
1. For variadic functions 'convertCallArguments' was allocating exactly
'len(args)' arguments, which might be less than required, as a result
zero/default filling loop would panic with out of bounds error.
2. Even if we correctly allocate a proper amount of arguments, zero/default
filling loop doesn't handle special variadic function type case, when last
argument has a slice type and reflect API expects plain values as arguments.
But actually we don't need that, because it's okay to omit variadic values
altogether, hence the total amount of allocated arguments is
max(len(args), rt.NumIn()-1).
