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
Change-Id: Icd28f63594ad941eabb3a12c750a4a2d5d2bf94b