PR22239 - invalid memory read in display_debug_frames

PR22239 - invalid memory read in display_debug_frames

Pointer comparisons have traps for the unwary.  After adding a large
unknown value to "start", the test "start < end" depends on where
"start" is originally in memory.

	PR 22239
	* dwarf.c (read_cie): Don't compare "start" and "end" pointers
	after adding a possibly wild length to "start", compare the length
	to the difference of the pointers instead.  Remove now redundant
	"negative" length test.