Fix ServerHello EC point format extension check.

Use the newly split out tls1_check_point_format. Also don't condition it on
s->tlsext_ecpointformatlist which is unrelated and made this code never run.

Change-Id: I9d77654c8eaebde07079d989cd60fbcf06025d75
Reviewed-on: https://boringssl-review.googlesource.com/1844
Reviewed-by: Adam Langley <agl@google.com>