ops: fix i<c-r> with multi-byte text (#6524)

test/functional/insert/ctrl_r_spec.lua

@@ -0,0 +1,19 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear, feed = helpers.clear, helpers.feed
+local expect, command = helpers.expect, helpers.command
+
+describe('insert-mode Ctrl-R', function()
+  before_each(clear)
+
+  it('works', function()
+    command("let @@ = 'test'")
+    feed('i<C-r>"')
+    expect('test')
+  end)
+
+  it('works with multi-byte text', function()
+    command("let @@ = 'påskägg'")
+    feed('i<C-r>"')
+    expect('påskägg')
+  end)
+end)