Stop reseting init_num everywhere in the handshake loop.

This was needed because ssl3_get_message would get confused if init_num were
not set back to zero when reading the next message. However, ssl3_get_message
now treats init_num only as an output, not an input. (The message sending logic
and the individual handshake states still use it, so we can't get rid of it
altogether yet.)

I've kept the init_num reset at the start and end of the handshake loop alone
for now since that's more about initialization and cleanup. Though I believe
they too do not do anything.

Change-Id: I64bbdd82122498de32364e7edb3b00b166059ecd
Reviewed-on: https://boringssl-review.googlesource.com/7950
Reviewed-by: Adam Langley <agl@google.com>