drm/mediatek/dp: Fix spurious kfree()

drm_edid_to_sad() might return an error or just zero. If that is the
case, we must not free the SADs because there was no allocation in
the first place.

Fixes: dab12fa8 ("drm/mediatek/dp: fix memory leak on ->get_edid callback audio detection")
Signed-off-by: Michael Walle <mwalle@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20240604083337.1879188-1-mwalle@kernel.org/Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

drivers/gpu/drm/mediatek/mtk_dp.c

@@ -2073,9 +2073,15 @@ static const struct drm_edid *mtk_dp_edid_read(struct drm_bridge *bridge,
 		 */
 		const struct edid *edid = drm_edid_raw(drm_edid);
 		struct cea_sad *sads;
+		int ret;
 
-		audio_caps->sad_count = drm_edid_to_sad(edid, &sads);
-		kfree(sads);
+		ret = drm_edid_to_sad(edid, &sads);
+		/* Ignore any errors */
+		if (ret < 0)
+			ret = 0;
+		if (ret)
+			kfree(sads);
+		audio_caps->sad_count = ret;
 
 		/*
 		 * FIXME: This should use connector->display_info.has_audio from