Fix integer underflow in covr MPEG4 processing

When the 'chunk_data_size' variable is less than 'kSkipBytesOfDataBox', an
integer underflow can occur. This causes an extraordinarily large value to
be passed to MetaData::setData, leading to a buffer overflow.

Bug: 20923261
(cherry picked from commit 18b7e3cf)

Change-Id: Id2484cfe3fdce0c9ba57d2bded0cdf019d6fe935