Add OSSL_NELEM macro.

Add OSSL_NELEM macro to e_os.h to determine the number of elements in an
array.

Reviewed-by: Tim Hudson <tjh@openssl.org>

ssl/s3_lib.c

diff --git a/ssl/s3_lib.c b/ssl/s3_lib.c
index c2fddb8..e346c22 100644
--- a/ssl/s3_lib.c
+++ b/ssl/s3_lib.c
@@ -159,7 +159,7 @@
 
 const char ssl3_version_str[] = "SSLv3" OPENSSL_VERSION_PTEXT;
 
-#define SSL3_NUM_CIPHERS        (sizeof(ssl3_ciphers)/sizeof(SSL_CIPHER))
+#define SSL3_NUM_CIPHERS        OSSL_NELEM(ssl3_ciphers)
 
 /* list of available SSLv3 ciphers (sorted by id) */
 OPENSSL_GLOBAL const SSL_CIPHER ssl3_ciphers[] = {