lump processing: correct the handling of DEL lumps with identical offsets

	* for this corner case, only add the _delta_ lengths instead
(cherry picked from commit f9d9654e6df6c801f19f774e3960a8d6f806a1ca)
lumps processing: a couple of performance optimizations

Since we can only have LUMP_DEL or LUMP_NOP along the main lump chain,
reordering the switch statements will improve code performance.

This patch also enhances code readability and improves error reporting.
(cherry picked from commit 6f83e15bba66ca1d1f1437fb5afb1ad25a49c430)