Segments are validated in hdr_validate_segments. Gaps in segment keys
are detected when collecting offsets. But if an invalid segment is very
large, larger than count, it could happen that cryptsetup is unable to
allocate enough memory, not giving a clue about what actually is the
problem.
Therefore check for gaps even if not enough memory is available. This
gives much more information with debug output enabled.
Obviously cryptsetup still fails if segments are perfectly fine but not
enough RAM available. But at that stage, the user knows that it's the
fault of the system, not of an invalid segment.
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