FKeyHandle internal static atomic counter can overflow on long running games depending on usage pattern

Structure FKeyHandle stores an internal index, which is initialized from a static atomic counter in the default constructor:

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Although an overflow seems unlikely, several Get() and Find() functions can return a default-constructed FKeyHandle() to represent an invalid handle, which increases the counter. If such functions are used frequently (e.g. multiple times every frame), overflowing can become a real possibility on reasonably long running games.

Interestingly, a special static function FKeyHandle::Invalid() exists, which returns a handle with invalid internal index 0 without incrementing the counter. A comment in file "KeyHandle.h" mentions that this is to "avoid allocating new handles unnecessarily". However, that function is not used in several places where it might be appropriate, such as:

[Engine\Source\Runtime\Engine\Private\Curves\IntegralCurve.cpp]
FIntegralCurve::FindKey()
FIntegralCurve::FindKeyBeforeOrAt()

[Engine\Source\Runtime\Engine\Classes\Animation\AttributeCurve.cpp]
//FAttributeCurve::FindKey() // This does use FKeyHandle::Invalid()!
FAttributeCurve::FindKeyBeforeOrAt()

[Engine\Source\Runtime\Engine\Private\Curves\NameCurve.cpp]
FNameCurve::FindKey()

[Engine\Source\Runtime\Engine\Private\Curves\StringCurve.cpp]
FStringCurve::FindKey()

[Engine\Source\Runtime\Engine\Public\Curves\KeyFrameManipulator.h]
TKeyFrameManipulator::GetKeyHandleFromIndex()

[Engine\Source\Runtime\MovieScene\Private\Channels\MovieSceneChannelData.cpp]
FMovieSceneChannelData::GetHandle()

[Engine\Source\Runtime\MovieScene\Public\Channels\MovieSceneChannel.h]
FMovieSceneChannel::GetHandle()

[Engine\Source\Editor\UnrealEd\Private\SCurveEditor.cpp]
SCurveEditor::HitTestKeys()

[Engine\Source\Editor\UnrealEd\Public\SCurveEditor.h]
FSelectedTangent::FSelectedTangent()

Note: This was reported by a licensee on an actual in-development game (see linked EPS case).

Ticket was generated from 3rd party UDN Vendor report via Slack-bot and is pending pre-processing.

Implement and execute a native function containing the following code:

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After several seconds (probably less than a minute), the index of the handle returned by FIntegralCurve::FindKeyBeforeOrAt() will wrap around and hit the check() in the constructor. On a non development build or when continuing past the check(), the function above will end up finding another FKeyHandle incorrectly considered equal to "HandleTo2000".