Reduce the scope of some variables in tls_process_client_key_exchange()

In preparation for splitting this function up into smaller functions this
commit reduces the scope of some of the variables to only be in scope for
the algorithm specific parts. In some cases that makes the error handling
more verbose than it needs to be - but we'll clean that up in a later
commit.

Reviewed-by: Richard Levitte <levitte@openssl.org>