copy: protect against overlapping extents

copy: protect against overlapping extents

* src/extent-scan.c (extent_scan_read): Add a more stringent check
for OFF_T overflow, to ensure subsequent code is immune.
Detect overlapping extents and adjust, so as files always copied.
Detection using a single scan with fallback to a standard copy
was thought too expensive in memory or time.
* NEWS: Mention the fix