Fixed a crash when opening JPEG files edited in Lightroom

We do not support nested arrays/nested structures in the XMP currently.
This means that all the information about Adjustment masks applied by
the Adobe Lightroom will be lost.

This is not ideal of course, but we can live with it for now.

Here is an example of XMP tag that will be dropped:
GradientBasedCorrections[1]/crs:CorrectionMasks[1]/crs:What