patch 8.2.4646: using buffer line after it has been freed

Problem:    Using buffer line after it has been freed in old regexp engine.
Solution:   After getting mark get the line again.

src/testdir/test_regexp_latin.vim

@@ -1042,10 +1042,17 @@ endfunc
 
 func Test_using_mark_position()
   " this was using freed memory
+  " new engine
   new
   norm O0
   call assert_fails("s/\\%')", 'E486:')
   bwipe!
+
+  " old engine
+  new
+  norm O0
+  call assert_fails("s/\\%#=1\\%')", 'E486:')
+  bwipe!
 endfunc
 
 func Test_using_visual_position()