meson: Disable asserts in release builds

Sets the meson b_ndebug option to default to if-release,
so that asserts are disabled in release builds.

meson.build

@@ -26,7 +26,8 @@ project('dav1d', ['c'],
     version: '0.0.1',
     default_options: ['c_std=c99',
                       'warning_level=2',
-                      'buildtype=release'],
+                      'buildtype=release',
+                      'b_ndebug=if-release'],
     meson_version: '>= 0.47.0')
 
 dav1d_version_array    = meson.project_version().split('.')