Touch up ssl3_get_message.

The |skip_message| variable was overly complex and, since we have at
least 32-bit ints, we know that a 24-bit value doesn't overflow an int.

Change-Id: I5c16fa979e1716f39cc47882c033bcf5bce3284c
Reviewed-on: https://boringssl-review.googlesource.com/2610
Reviewed-by: Adam Langley <agl@google.com>