fix(decoration): fix crash when on_lines decor provider modifies marktree

If a "on_lines" callback changes the structure of the marktree, the
iterator (which is used for an entire window viewport) might now
point to invalid memory. Restore the iterator to the beginning of the
line in this case.

fixes #29484

test/functional/ui/decorations_spec.lua

@@ -744,6 +744,30 @@ describe('decorations providers', function()
     ]])
     eq(2, exec_lua([[return _G.cnt]]))
   end)
+
+  it('can do large changes to the marktree', function()
+    insert("line1 with a lot of text\nline2 with a lot of text")
+    setup_provider([[
+      function on_do(event, _, _, row)
+        if event == 'win' or (event == 'line' and row == 1) then
+          vim.api.nvim_buf_clear_namespace(0, ns1, 0, -1)
+          for i = 0,1 do
+            for j = 0,23 do
+              vim.api.nvim_buf_set_extmark(0, ns1, i, j, {hl_group='ErrorMsg', end_col = j+1})
+            end
+          end
+        end
+      end
+    ]])
+
+    -- Doesn't crash when modifying the marktree between line1 and line2
+    screen:expect([[
+      {2:line1 with a lot of text}                |
+      {2:line2 with a lot of tex^t}                |
+      {1:~                                       }|*5
+                                              |
+    ]])
+  end)
 end)
 
 local example_text = [[