Add a null check in RTSPSource::stop

This was causing a null pointer dereference in stop.

Manual backport of aosp/1456267.

Bug: 170643945
Bug: 177500688
Test: backport of aosp/1456267, tested in CTS
Change-Id: I8c771cd68663ebb9df79584ba5dd4edb6636bdbd
1 file changed