Remove a few redundant memset after calloc.

Using the following semantic patch provided by Mansour Moufid:

@@
expression x;
@@
  x = mbedtls_calloc(...)
  ...
- memset(x, 0, ...);

library/x509_crl.c

@@ -243,7 +243,6 @@ static int x509_get_entries( unsigned char **p,
             if( cur_entry->next == NULL )
                 return( MBEDTLS_ERR_X509_MALLOC_FAILED );
 
-            memset( cur_entry->next, 0, sizeof( mbedtls_x509_crl_entry ) );
             cur_entry = cur_entry->next;
         }
     }