Various clean-ups

Add a check for NULL return in t1_lib.c.
    Since return type of ssl_cert_lookup_by_idx is pointer and unify coding
    style, I changed from zero to NULL in ssl_cert.c.

Remove unnecessary space for ++.

Fix incorrect condition
    Expression is always false because 'else if' condition matches previous
    condition.  SInce the next line of 'else if' condition has substituted
    TLSEXT_ECPOINTFORMAT_ansiX962_compressed_char2, the 'else if'
    condition should compare with NID_X9_62_characteristic_two_field.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4562)