Fix 22838 - BitArray.count reads beyond data

[JohanEngelen]

This fixes issue 22838
https://issues.dlang.org/show_bug.cgi?id=22838

All hail AddressSanitizer! ;-)

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	18036	normal	Documentation of moveFront() fails to mention different behavior depending on hasElaborateCopyConstructor
✗	21975	normal	is expression ignores implicit conversion of struct via alias this when pattern matching
✓	22771	normal	BigInt divMod can return "-0" (negative zero)
✓	22791	normal	std\socket.d(790) Heisenbug random failure
✓	22798	enhancement	defaultGetoptPrinter should be @safe
✓	22838	critical	std.bitmanip.BitArray.count() reads beyond data when data size is integer size_t multiple

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "stable + phobos#8400"

[JohanEngelen]

Closing, resubmitting on stable

[JohanEngelen]

#8401