Fix SEGV in StructuredHeaders::decodeBase64

Summary:
The existing code can potentially cause a SEGV due to an out of bounds write.

This fixes CVE-2019-11921.

Reviewed By: knekritz

Differential Revision: D12983120

fbshipit-source-id: 1d48063595c8d518fd8afcbc941de260af7e37fd