drm/amd/display: Skip finding free audio for unknown engine_id

[WHY]
ENGINE_ID_UNKNOWN = -1 and can not be used as an array index. Plus, it
also means it is uninitialized and does not need free audio.

[HOW]
Skip and return NULL.

This fixes 2 OVERRUN issues reported by Coverity.

Reviewed-by: Rodrigo Siqueira <[email protected]>
Acked-by: Wayne Lin <[email protected]>
Signed-off-by: Alex Hung <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>

Author: Alex Hung

drivers/gpu/drm/amd/display/dc/core/dc_resource.c

@@ -3187,6 +3187,9 @@ static struct audio *find_first_free_audio(
 {
 	int i, available_audio_count;
 
+	if (id == ENGINE_ID_UNKNOWN)
+		return NULL;
+
 	available_audio_count = pool->audio_count;
 
 	for (i = 0; i < available_audio_count; i++) {