Original commit message from CVS:
Make GST_PROPS_PROPERTIES return NULL when props is NULL. This allows
GST_PROPS_PROPERTIES (gst_caps_get_props (caps)) even if the caps have no props.
#define GST_PROPS_FLAG_UNSET(props,flag) (GST_PROPS_FLAGS (props) &= ~(flag))
#define GST_PROPS_REFCOUNT(props) ((props)->refcount)
-#define GST_PROPS_PROPERTIES(props) ((props)->properties)
+#define GST_PROPS_PROPERTIES(props) ((props) ? ((props)->properties) : NULL)
#define GST_PROPS_IS_FIXED(props) (GST_PROPS_FLAG_IS_SET ((props), GST_PROPS_FIXED))
#define GST_PROPS_IS_FLOATING(props) (GST_PROPS_FLAG_IS_SET ((props), GST_PROPS_FLOATING))