Add and use OPENSSL_zalloc

There are many places (nearly 50) where we malloc and then memset.
Add an OPENSSL_zalloc routine to encapsulate that.
(Missed one conversion; thanks Richard)
Also fixes GH328

Reviewed-by: Richard Levitte <levitte@openssl.org>
diff --git a/test/ssltest.c b/test/ssltest.c
index 9144191..adf1368 100644
--- a/test/ssltest.c
+++ b/test/ssltest.c
@@ -2116,14 +2116,11 @@
 
     bufsiz = count > 40 * 1024 ? 40 * 1024 : count;
 
-    if ((cbuf = OPENSSL_malloc(bufsiz)) == NULL)
+    if ((cbuf = OPENSSL_zalloc(bufsiz)) == NULL)
         goto err;
-    if ((sbuf = OPENSSL_malloc(bufsiz)) == NULL)
+    if ((sbuf = OPENSSL_zalloc(bufsiz)) == NULL)
         goto err;
 
-    memset(cbuf, 0, bufsiz);
-    memset(sbuf, 0, bufsiz);
-
     c_to_s = BIO_new(BIO_s_mem());
     s_to_c = BIO_new(BIO_s_mem());
     if ((s_to_c == NULL) || (c_to_s == NULL)) {