GMemoryInputStream: fix skip_async()

GMemoryInputStream: fix skip_async()

a5876e5f made GMemoryInputStream subclassable, but accidentally broke
read_async() and skip_async() in the process. The immediately
following e7983495 fixed read_async() (and added a test for it), but
skip_async() accidentally got... skipped.

Fix it now and add a test for it.

Also, GMemoryInputStream's skip_async() was assuming that skip() could
never fail, which is true of its own implementation, but might not be
true of a subclass's, so do proper GError handling too.