This removes the last place where non-app-data hooks leave anything
uncomsumed in rrec. (There is still a place where non-app-data hooks see
a non-empty rrec an entrance. read_app_data calls into read_handshake.
That'll be fixed in a later patch in this series.)

This should not change behavior, though some error codes may change due
to some processing happening in a slightly different order.

Since we do this in a few places, this adds a BUF_MEM_append with tests.

Change-Id: I9fe1fc0103e47f90e3c9f4acfe638927aecdeff6
Reviewed-on: https://boringssl-review.googlesource.com/21345
Commit-Queue: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Steven Valdez <svaldez@google.com>