bugfix: infinite loop inside a very narrow window

I'm not sure this can trigger everywhere (I've only been able to
exercise it in Lua Carousel), but it seems like a safety net worth
having against future modifications by anybody.

source_text.lua

@@ -128,7 +128,9 @@ function Text.populate_screen_line_starting_pos(State, line_index)
         -- long word; chop it at some letter
         -- We're not going to reimplement TeX here.
         local bpos = Text.nearest_pos_less_than(frag, State.width - x)
-        -- everything works if bpos == 0, but is a little inefficient
+        if x == 0 and bpos == 0 then
+          assert(false, ("Infinite loop while line-wrapping. Editor is %dpx wide; window is %dpx wide"):format(State.width, App.screen.width))
+        end
         pos = pos + bpos
         local boffset = Text.offset(frag, bpos+1)  -- byte _after_ bpos
         frag = string.sub(frag, boffset)

text.lua

@@ -103,7 +103,9 @@ function Text.populate_screen_line_starting_pos(State, line_index)
         -- long word; chop it at some letter
         -- We're not going to reimplement TeX here.
         local bpos = Text.nearest_pos_less_than(frag, State.width - x)
-        -- everything works if bpos == 0, but is a little inefficient
+        if x == 0 and bpos == 0 then
+          assert(false, ("Infinite loop while line-wrapping. Editor is %dpx wide; window is %dpx wide"):format(State.width, App.screen.width))
+        end
         pos = pos + bpos
         local boffset = Text.offset(frag, bpos+1)  -- byte _after_ bpos
         frag = string.sub(frag, boffset)