The handshake state machine is still rather messy (we should switch to CBB,
split the key exchanges apart, and also pull reading and writing out), but this
version makes it more obvious to the compiler that |p| and |sig_len| are
initialized. The old logic created a synchronous-only state which, if enterred
directly, resulted in some variables being uninitialized.

Change-Id: Ia3ac9397d523fe299c50a95dc82a9b26304cea96
Reviewed-on: https://boringssl-review.googlesource.com/5765
Reviewed-by: Adam Langley <agl@google.com>