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baseparse: Don't return more data than asked for in pull_range()

Even when pulling a new 64KB buffer from upstream, don't return
more data than was asked for in the pull_range() method and then
return less later, as that confused subclasses like h264parse.

Add a unit test that when a subclass asks for more data, it always
receives a larger buffer on the next iteration, never less.

Fixes #530

Part-of: <!733>