Fix bug #13743 with crashes due to recursive add-text-properties.
src/textprop.c (Fadd_text_properties, Fremove_text_properties) (Fremove_list_of_text_properties): Skip all of the intervals in the region between START and END that already have resp. don't have the requested properties, not just the first one. Add assertions that the loop afterwards always modifies the properties.
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