Standardise our style for checking malloc failures

if we have a malloc |x = OPENSSL_malloc(...)| sometimes we check |x|
for NULL and sometimes we treat it as a boolean |if(!x) ...|. Standardise
the approach in libssl.

Reviewed-by: Kurt Roeckx <kurt@openssl.org>

ssl/ssl_ciph.c

@@ -1039,7 +1039,7 @@ static int ssl_cipher_strength_sort(CIPHER_ORDER **head_p,
     }
 
     number_uses = OPENSSL_zalloc(sizeof(int) * (max_strength_bits + 1));
-    if (!number_uses) {
+    if (number_uses == NULL) {
         SSLerr(SSL_F_SSL_CIPHER_STRENGTH_SORT, ERR_R_MALLOC_FAILURE);
         return (0);
     }