When handling PART_KIND_FILE, the "error" variable is set to reflect
any errors in parsing the specified offset; but that value is then
overwritten by the return value of image_copyin.
Error out immediately instead. Note that it is not necessary to change
the code in PART_KIND_SIZE which parses the size, since in that case we
do not overwrite the value in "error".
Fixes: 8f3c71c85e5f ("mkimg: Add support for offset if the source is an image")
MFC after: 1 week
Sponsored by: Amazon