Check segment gaps regardless of heap space.

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.