buffer: add gst_clear_buffer()

Basically, you can use this instead of using gst_buffer_unref (which
needs to be preceded by a NULL-check).

Also fixes #275

gst/gstbuffer.h

@@ -442,6 +442,25 @@ gst_buffer_unref (GstBuffer * buf)
   gst_mini_object_unref (GST_MINI_OBJECT_CAST (buf));
 }
 
+/**
+ * gst_clear_buffer: (skip)
+ * @buf_ptr: a pointer to a #GstBuffer reference
+ *
+ * Clears a reference to a #GstBuffer.
+ *
+ * @buf_ptr must not be %NULL.
+ *
+ * If the reference is %NULL then this function does nothing. Otherwise, the
+ * reference count of the buffer is decreased and the pointer is set to %NULL.
+ *
+ * Since: 1.16
+ */
+static inline void
+gst_clear_buffer (GstBuffer ** buf_ptr)
+{
+  gst_clear_mini_object (buf_ptr);
+}
+
 /* copy buffer */
 /**
  * gst_buffer_copy: