patch 8.2.1939: invalid memory access in Ex mode with global command

Problem:    Invalid memory access in Ex mode with global command.
Solution:   Make sure the cursor is on a valid line. (closes #7238)

src/testdir/test_ex_mode.vim

@@ -1,6 +1,7 @@
 " Test editing line in Ex mode (see :help Q and :help gQ).
 
 source check.vim
+source shared.vim
 
 " Helper function to test editing line in Q Ex mode
 func Ex_Q(cmd)
@@ -179,4 +180,28 @@ func Test_ex_mode_errors()
   quit
 endfunc
 
+func Test_ex_mode_with_global()
+  CheckFeature timers
+
+  " This will get stuck in Normal mode after the failed "J", use a timer to
+  " get going again.
+  let lines =<< trim END
+    call ch_logfile('logfile', 'w')
+    pedit
+    func FeedQ(id)
+      call feedkeys('Q', 't')
+    endfunc
+    call timer_start(10, 'FeedQ')
+    g/^/vi|HJ
+    call writefile(['done'], 'Xdidexmode')
+    qall!
+  END
+  call writefile(lines, 'Xexmodescript')
+  call assert_equal(1, RunVim([], [], '-e -s -S Xexmodescript'))
+  call assert_equal(['done'], readfile('Xdidexmode'))
+
+  call delete('Xdidexmode')
+  call delete('Xexmodescript')
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab