enforce minimum FileTypeBox length in Jp2Image::printStructure (backport #9318)

[mergify]

exiv2 -pS on a JP2 whose FileTypeBox sets its length below the 8-byte box header:

   12 |        2 | ftyp      | Uncaught exception: vector

box.length - boxHSize underflows, so Blob is sized to near SIZE_MAX and std::vector throws std::length_error rather than an Exiv2::Error. The sibling sub-box and uuid cases already guard length; this adds the same check before the FileTypeBox allocation.

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This is an automatic backport of pull request #9318 done by [Mergify](https://mergify.com).

[mergify]

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	both modified:   src/jp2image.cpp

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