MetadataRetriever: Check null pointer to avoid tombstone crash.

When the app's vm-heap is not enough, memory allocation for big
object like bitmap may failed.

This patch add protection for bitmap creating to avoid segment
fault error in the following GetIntField function.

Change-Id: I63977dc602f4ed395afd11004a0ed027173fb685
Signed-off-by: wang, biao <biao.wang@intel.com>
Signed-off-by: TingX Li <tingx.li@intel.com>
Signed-off-by: Wang LiangX <liangx.wang@intel.com>