lib: return NAN on libvlc_audio_equalizer_set_amp_at_index() error

include/vlc/libvlc_media_player.h

@@ -1833,7 +1833,7 @@ LIBVLC_API int libvlc_audio_equalizer_set_amp_at_index( libvlc_equalizer_t *p_eq
  *
  * \param p_equalizer valid equalizer handle, must not be NULL
  * \param u_band index, counting from zero, of the frequency band to get
- * \return amplification value (Hz); zero if there is no such frequency band
+ * \return amplification value (Hz); NaN if there is no such frequency band
  * \version LibVLC 2.2.0 or later
  */
 LIBVLC_API float libvlc_audio_equalizer_get_amp_at_index( libvlc_equalizer_t *p_equalizer, unsigned u_band );

lib/audio.c

@@ -621,7 +621,7 @@ int libvlc_audio_equalizer_set_amp_at_index( libvlc_equalizer_t *p_equalizer, fl
 float libvlc_audio_equalizer_get_amp_at_index( libvlc_equalizer_t *p_equalizer, unsigned u_band )
 {
     if ( u_band >= EQZ_BANDS_MAX )
-        return 0.f;
+        return nanf("");
 
     return p_equalizer->f_amp[ u_band ];
 }