[stdlib][runtime] fix uninitialized variable savedSize

[bryonglodencissp]

On line number 2560 of Metadata.cpp, savedSize is accessed without an initialized value. It's always good to initialize variables, because if we try to access a value before it gets assigned any actual (non-garbage) value, then it results in undefined behavior.

Found by https://github.com/bryongloden/cppcheck

[gottesmm]

I am not sure if this is actually undefined behavior. The value is being captured by reference in the lambda and is initialized strictly before the lambda runs. @rjmccall/@DougGregor what do you guys think?

[bryonglodencissp]

It's also worth noting that we've done extensive studies, and 60% of the time our static analysis tool works all the time. Furthermore, sound and complete static analysis is shown to be undecidable, ergo we're led to a few false positives because we've adopted some unsound techniques in our tool.

[gottesmm]

I talked with Doug/etc and we are pretty sure this is not undefined behavior. But that being said, all it would take is a little refactoring to introduce the undefined behavior and also we appreciate you running the tool (so if it quiets the error sounds good).

[gottesmm]

@swift-ci Please smoke test and merge