Always invoke DecodeNameCycle with the length arg

Signed-off-by: Fons Rademakers <Fons.Rademakers@cern.ch>
Turned CID 53502 into a warning user message

DecodeNameCycle() is now safe against buffer overflows: in order to
keep it backwards-compatible, an "unsafe" use mode has been retained
and triggered a Coverity notification. The "unsafe mode" now does not
trigger Coverity but instead warns the user via an on screen message.

Signed-off-by: Fons Rademakers <Fons.Rademakers@cern.ch>