completion: fix segfault with ignorecase+infercase (#6452)

Helped-by: Matthew Malcomson <hardenedapple@gmail.com>

Closes #6451

test/functional/viml/completion_spec.lua

@@ -850,6 +850,22 @@ describe('completion', function()
       ]])
     end)
   end)
+
+  it("'ignorecase' 'infercase' CTRL-X CTRL-N #6451", function()
+    execute('set ignorecase infercase')
+    execute('edit BACKERS.md')
+    feed('oX<C-X><C-N>')
+    screen:expect([[
+      # Bountysource Backers                                      |
+      Xnull^                                                       |
+      {2:Xnull          }{6: }                                            |
+      {1:Xoxomoon       }{6: }ryone who backed our [Bountysource fundraise|
+      {1:Xu             }{6: }ountysource.com/teams/neovim/fundraiser)!   |
+      {1:Xpayn          }{2: }                                            |
+      {1:Xinity         }{2: }d URL in BACKERS.md.                        |
+      {3:-- Keyword Local completion (^N^P) }{4:match 1 of 7}             |
+    ]])
+  end)
 end)
 
 describe('External completion popupmenu', function()