mpeg2_parser: check for nalunit size before starting bitReader
mpeg2_parser: check for nalunit size before starting bitReader

This is to avoid a potential memory leak when the stream size is not
large enough and function has to return.  bitReader can be created after
such check

Signed-off-by: Daniel Charles <9f8d44a34cf25995df1daf4e48c6fa87b67d5dc5@intel.com>

YuJiankang/libyami,dspmeng/libyami,uartie/libyami,uartie/libyami,stripes416/libyami,zhaobob/libyami,uartie/libyami,01org/libyami,lizhong1008/libyami,seanvk/libyami,dspmeng/libyami,lizhong1008/libyami,stripes416/libyami,stripes416/libyami,zhaobob/libyami,seanvk/libyami,yizhouwei/libyami,01org/libyami,seanvk/libyami,dspmeng/libyami,yizhouwei/libyami,zhaobob/libyami,YuJiankang/libyami,yizhouwei/libyami,01org/libyami,YuJiankang/libyami,lizhong1008/libyami