[ec/ecp_nistp*.c] restyle: use {} around `else` too

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/9511)

(cherry picked from commit 4fe2ee3a449a8ca2886584e221f34ff0ef5de119)