[Issue 19338] std.bitmanip.BitArray.count gives segfault for empy BitArray

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19338

--- Comment #1 from Richard Palme <richardpalme5b at gmail.com> ---
I forgot to mention: the expected return value of b.count for an empty BitArray
b would be 0, similar to how b.length returns 0 for empty BitArray b.

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