compositor-drm: drm_property_info_free() must reset

This function needs to reset the structures to NULL, otherwise it is not
possible to re-use a once "freed" property info array.

Being able to re-use an array is useful when the memory allocation and
array lifetimes do not match. A specific example is drm_output that is
changed to allocate the CRTC on enable() and deallocate it on disable().
A drm_output might be enabled and disabled multiple times.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

libweston/compositor-drm.c

@@ -760,7 +760,9 @@ drm_property_info_populate(struct drm_backend *b,
 /**
  * Free DRM property information
  *
- * Frees all memory associated with a DRM property info array.
+ * Frees all memory associated with a DRM property info array and zeroes
+ * it out, leaving it usable for a further drm_property_info_update() or
+ * drm_property_info_free().
  *
  * @param info DRM property info array
  * @param num_props Number of entries in array to free
@@ -772,6 +774,8 @@ drm_property_info_free(struct drm_property_info *info, int num_props)
 
 	for (i = 0; i < num_props; i++)
 		free(info[i].enum_values);
+
+	memset(info, 0, sizeof(*info) * num_props);
 }
 
 static void