Zero s->packet when releasing the read buffer.

s->packet points into the read buffer. It shouldn't leave a dangling pointer.

Change-Id: Ia7def2f50928ea9fca8cb0b69d614a92f9f47f57
Reviewed-on: https://boringssl-review.googlesource.com/4684
Reviewed-by: Adam Langley <agl@google.com>