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>

crypto/rsa/rsa_pk1.c

@@ -203,12 +203,11 @@ int RSA_padding_check_PKCS1_type_2(unsigned char *to, int tlen,
     if (num < 11)
         goto err;
 
-    em = OPENSSL_malloc(num);
+    em = OPENSSL_zalloc(num);
     if (em == NULL) {
         RSAerr(RSA_F_RSA_PADDING_CHECK_PKCS1_TYPE_2, ERR_R_MALLOC_FAILURE);
         return -1;
     }
-    memset(em, 0, num);
     /*
      * Always do this zero-padding copy (even when num == flen) to avoid
      * leaking that information. The copy still leaks some side-channel