[SR-6361] Fix Data.withUnsafeMutableBytes() for slices with length < range.lowerBound

[sashabelonogov]

Solves SR-6361
This code snippet:

var data = Data(bytes: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9])[4..<6]
data.withUnsafeMutableBytes { (ptr: UnsafeMutablePointer<UInt8>) in
    ptr.advanced(by: 1).pointee = 0xFF
}

crashes with:
fatal error: UnsafeMutableRawBufferPointer with negative count

because count was not calculated correctly (_offset was not taken into account)

The existing tests were not covering this case because it works correctly for data slices when _length > range.lowerBound.

I created new tests for this exact situation but I can also update the existing ones by adding one more check to them.

Please, let me know what you think.

[spevans]

@swift-ci please test

[pingchen114]

Just curious, maybe Data.withUnsafeBytes() followed the same logic and should be fixed too?

[alblue]

If you finish this line with a comma (like in like 511) then others won't need to change line 517 when they add more tests in the future.

[sashabelonogov]

Thanks @alblue, I updated the commit

[alblue]

@phausler the tests pass on this; do you think the change is reasonable?

[sashabelonogov]

@my0114 thanks for your feedback! You might be right, I will check it in a couple of hours to make sure.

[spevans]

@swift-ci please test

[spevans]

@swift-ci please test

[phausler]

Good catch, can you please put up this as a PR on the overlay as well?

[sashabelonogov]

thanks @phausler, sure, this is the PR: swiftlang/swift#12898

[sashabelonogov]

@phausler I'm sorry for changing this PR after review but I tried to investigate the issue further and realized that simple count: Swift.min(range.count, _length))) is simpler and will do the job, so I updated both PRs.
I hope I understood everything correctly and it makes sense. Thanks.