sd: fix race condition in vlc_media_source_provider_GetMediaSource

T1:

• calls vlc_media_source_Release
• rc is decremented and equals 0
• start calling vlc_media_source_Delete (provider->lock is not locked yet)

T2:

• calls vlc_media_source_provider_GetMediaSource
• locks provider->lock
• vlc_media_source_provider_Find will return the node (not yet removed from the list)
• vlc_media_source_Hold increments rc, assert as rc == 0

fix: #28891 (closed)