Fix static analysis detected error regarding undefined array index in dvdringbuf.cpp

Static analysis (scan-build) detected a potential code path such that guess_palette
could be called without having the palette variables initialized, which would result
in indexing m_clut with an undefined value.
(cherry picked from commit 824013456f41f4c90daa1f1a3879a1c9e06c28c5)

Signed-off-by: Stuart Morgan <smorgan@mythtv.org>

Fixes #11550