rtsp: Fix (what I think may be) a minor bug.

      (Also fixes a compiler 'Shadow' warning in a different way
       than the now reverted previous fix).

Change-Id: Iedcac3d402b89d1b0261a49647bf39cc854d1a0c
Reviewed-on: https://code.wireshark.org/review/6123
Reviewed-by: Bill Meier <wmeier@newsguy.com>