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/crypto/dso/dso_lib.c b/crypto/dso/dso_lib.c
index 12544b3..17d1732 100644
--- a/crypto/dso/dso_lib.c
+++ b/crypto/dso/dso_lib.c
@@ -104,12 +104,11 @@
          */
         default_DSO_meth = DSO_METHOD_openssl();
     }
-    ret = OPENSSL_malloc(sizeof(*ret));
+    ret = OPENSSL_zalloc(sizeof(*ret));
     if (ret == NULL) {
         DSOerr(DSO_F_DSO_NEW_METHOD, ERR_R_MALLOC_FAILURE);
         return (NULL);
     }
-    memset(ret, 0, sizeof(*ret));
     ret->meth_data = sk_void_new_null();
     if (ret->meth_data == NULL) {
         /* sk_new doesn't generate any errors so we do */