patch 9.1.0447: completion may be wrong when deleting all chars

Problem:  completion may be wrong when deleting all chars
Solution: reset compl_shown_match

closes: #14854

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_ins_complete.vim b/src/testdir/test_ins_complete.vim
index eb89a15..d1b9609 100644
--- a/src/testdir/test_ins_complete.vim
+++ b/src/testdir/test_ins_complete.vim
@@ -630,14 +630,14 @@
   CheckScreendump
 
   let lines =<< trim END
-      funct Omni_test(findstart, base)
-	if a:findstart
-	  return col(".") - 1
-	endif
-	return [#{word: "one", info: "1info"}, #{word: "two", info: "2info"}, #{word: "three", info: "3info"}]
-      endfunc
-      set omnifunc=Omni_test
-      set completeopt+=longest
+    func Omni_test(findstart, base)
+      if a:findstart
+        return col(".") - 1
+      endif
+      return [#{word: "one", info: "1info"}, #{word: "two", info: "2info"}, #{word: "three", info: "3info"}]
+    endfunc
+    set omnifunc=Omni_test
+    set completeopt+=longest
   END
 
   call writefile(lines, 'Xpreviewscript', 'D')
diff --git a/src/testdir/test_popup.vim b/src/testdir/test_popup.vim
index f5cb8b2..1142efc 100644
--- a/src/testdir/test_popup.vim
+++ b/src/testdir/test_popup.vim
@@ -1175,6 +1175,8 @@
   set completeopt=menu,menuone
   call feedkeys("i\<C-X>\<C-O>\<BS>\<BS>\<BS>f", 'tx')
   call assert_equal('five', g:word)
+  call feedkeys("i\<C-X>\<C-O>\<BS>\<BS>\<BS>f\<BS>", 'tx')
+  call assert_equal('one', g:word)
 
   autocmd! AAAAA_Group
   set complete& completeopt&