Fix a crash due to X264_init() setting the libx264 rc_eq field to NULL
when rc_eq was not defined. As a consequence rc_eq can no longer be
manually specified and passed to libx264, but this option is going to
be removed from the libx264 public API anyway.

See the thread: "Fix crash in libx264 when rc_eq is not set".

Originally committed as revision 15029 to svn://svn.ffmpeg.org/ffmpeg/trunk