The size of an int is 4, not 2. To avoid worrying about this, add a GetVector
counterpart to GetString that handles all this. Apply this uniformly to avoid
all the pointer casts. This is less important for vector<uint8_t>, but even
then we'll now notice a 1-byte OOB read since std::string is NUL-terminated.
Also it's shorter.
Change-Id: Ie96591cb8d8d52742f5fd30d70b6af0511109585
Reviewed-on: https://boringssl-review.googlesource.com/30864
Reviewed-by: Adam Langley <agl@google.com>