Narrow the scope of local variables in tls_construct_client_key_exchange()
Narrow the scope of local variables in tls_construct_client_key_exchange()

This is in preparation for splitting up this over long function.

Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl