Feed Import stalled/stopped in Heap.pike

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=7644

Reported by Erik Allemann erik@roxen.com

RT#26577 & RT#26578
REP 6.0.92

Feed Import had stopped. Debug logged repeatedly reports:

 2d 8h 8m : Feed Import: periodic_scan threw an error:
          : Indexing the NULL value with "value".
          : pike/lib/modules/ADT.pmod/Heap.pike:198: ADT.Heap()->peek()
          : pike/lib/modules/Filesystem.pmod/Monitor.pmod/basic.pike:1485: RoxenModule(EP/feed-import#0)->feed_monitor->monitor->check(UNDEFINED,1000,
          : packages/feedimport/modules/feed-import.pike (564e550f):1508: RoxenModule(EP/feed-import#0)->feed_monitor->monitor->check(UNDEFINED,1000,@
12:09:10  : packages/feedimport/modules/feed-import.pike (564e550f):2231: RoxenModule(EP/feed-import#0)->feed_monitor->periodic_scan()
 2d 8h 8m : packages/feedimport/modules/feed-import.pike (564e550f):2281: RoxenModule(EP/feed-import#0)->feed_monitor->periodic_scan_dispatch()