mkvparser: reject files w/multiple ProjectionPrivate
mkvparser: reject files w/multiple ProjectionPrivate

further validation of the Projection element could be added, this check
avoids a memory leak

Bug: b/148840606
Change-Id: I5dd7129970bc4df9d21f414865d1b7983493815e

webmproject/libwebm,webmproject/libwebm,webmproject/libwebm