patch 8.2.1807: can use :help in a terminal popup window

Problem:    Can use :help in a terminal popup window.
Solution:   Give an error. (closes #7088)
diff --git a/src/help.c b/src/help.c
index 4f2b6b6..4d59ab9 100644
--- a/src/help.c
+++ b/src/help.c
@@ -39,6 +39,9 @@
     int		old_KeyTyped = KeyTyped;
 #endif
 
+    if (ERROR_IF_ANY_POPUP_WINDOW)
+	return;
+
     if (eap != NULL)
     {
 	// A ":help" command ends at the first LF, or at a '|' that is
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index e991fc1..187b4f1 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -2645,6 +2645,10 @@
   let g:test_is_flaky = 1
 
   let origwin = win_getid()
+
+  " open help window to test that :help below fails
+  help
+
   let termbuf = term_start(&shell, #{hidden: 1})
   let winid = popup_create(termbuf, #{minwidth: 40, minheight: 10})
   " Wait for shell to start
@@ -2666,6 +2670,7 @@
 
   " Cannot escape from terminal window
   call assert_fails('tab drop xxx', 'E863:')
+  call assert_fails('help', 'E994:')
 
   " Cannot open a second one.
   let termbuf2 = term_start(&shell, #{hidden: 1})
@@ -2677,6 +2682,7 @@
   " Wait for shell to exit
   call WaitForAssert({-> assert_equal("dead", job_status(term_getjob(termbuf)))})
 
+  helpclose
   call feedkeys(":quit\<CR>", 'xt')
   call assert_equal(origwin, win_getid())
 endfunc
diff --git a/src/version.c b/src/version.c
index a410167..daac372 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1807,
+/**/
     1806,
 /**/
     1805,