Small fix in StaticStringBuilder
Small fix in StaticStringBuilder

Summary:
If someone used less than the full capacity of the stringbuilder, it would have
put garbage or crashed. This is more robust. Now, `str()` only reads strings
that were actually put in.

Reviewed By: emma0303

Differential Revision: D8397930

fbshipit-source-id: 8fcefa659b8abf4697f643ce089bc823b7570b24

facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex,facebook/redex