Move random function check

move to `ssl_conf_check`

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 07b5100..1c8c7bd 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -3182,6 +3182,12 @@
     if( ret != 0 )
         return( ret );
 
+    if( ssl->conf->f_rng == NULL )
+    {
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "no RNG provided" ) );
+        return( MBEDTLS_ERR_SSL_NO_RNG );
+    }
+
     /* Space for further checks */
 
     return( 0 );