qcow2: Fix header extension size check

qcow2: Fix header extension size check

After reading the extension header, offset is incremented, but not
checked against end_offset any more. This way an integer overflow could
happen when checking whether the extension end is within the allowed
range, effectively disabling the check.

This patch adds the missing check and a test case for it.

Cc: [email protected]
Reported-by: Max Reitz <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Message-id: 1416935562[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>