Resolve MSAN use-of-uninitialized-value

[acgetchell]

Fix #149

[jaredgrubb]

For any object type, C++ will invoke the constructor by default (it only doesn't do that if it's a plain-old-data and lacks a user-provided constructor). All STL types are safe to use in this way. If there's a MSAN bug, this isn't the right fix. We can follow up in the issue.