patch 8.2.4951: smart indenting done when not enabled

Problem:    Smart indenting done when not enabled.
Solution:   Check option values before setting can_si. (closes #10420)
diff --git a/src/testdir/test_smartindent.vim b/src/testdir/test_smartindent.vim
index 7b58af2..edfec9a 100644
--- a/src/testdir/test_smartindent.vim
+++ b/src/testdir/test_smartindent.vim
@@ -134,4 +134,21 @@
   bw!
 endfunc
 
+func Test_si_after_completion()
+  new
+  setlocal ai smartindent indentexpr=
+  call setline(1, 'foo foot')
+  call feedkeys("o  f\<C-X>\<C-N>#", 'tx')
+  call assert_equal('  foo#', getline(2))
+  bwipe!
+endfunc
+
+func Test_no_si_after_completion()
+  new
+  call setline(1, 'foo foot')
+  call feedkeys("o  f\<C-X>\<C-N>#", 'tx')
+  call assert_equal('  foo#', getline(2))
+  bwipe!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab