BUF_MEM is actually a rather silly API for the SSL stack. There's separate
length and max fields, but init_buf effectively treats length as max and max as
nothing.

We possibly don't want to be using it long-term anyway (if nothing else, the
char*/uint8_t* thing is irritating), but in the meantime, it'll be easier to
separately fix up get_message's book-keeping and state tracking from where the
handshake gets its messages from.

Change-Id: I9e56ea008173991edc8312ec707505ead410a9ee
Reviewed-on: https://boringssl-review.googlesource.com/7947
Reviewed-by: Adam Langley <agl@google.com>