Fix warnings from armcc
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 26476a3..bd010a4 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -5722,7 +5722,7 @@
     if( ssl->session_negotiate != NULL )
         return( ssl->session_negotiate->verify_result );
 
-    return( -1 );
+    return( 0xFFFFFFFF );
 }
 
 const char *mbedtls_ssl_get_ciphersuite( const mbedtls_ssl_context *ssl )