patch 8.2.1963: crash when using a popup window with "latin1" encoding

Problem:    Crash when using a popup window with "latin1" encoding.
Solution:   Don't use ScreenLinesUC when enc_utf8 is false. (closes #7241)
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 435873e..43dbeb5 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -3685,6 +3685,28 @@
   call delete('XtestPopupThreeErrors')
 endfunc
 
+func Test_popupwin_latin1_encoding()
+  CheckScreendump
+  CheckUnix
+
+  " When 'encoding' is a single-byte encoding a terminal window will mess up
+  " the display.  Check that showing a popup on top of that doesn't crash.
+  let lines =<< trim END
+      set encoding=latin1
+      terminal cat Xmultibyte
+      call popup_create(['one', 'two', 'three', 'four'], #{line: 1, col: 10})
+  END
+  call writefile(lines, 'XtestPopupLatin')
+  call writefile([repeat("\u3042 ", 120)], 'Xmultibyte')
+
+  let buf = RunVimInTerminal('-S XtestPopupLatin', #{rows: 10})
+
+  call term_sendkeys(buf, ":q\<CR>")
+  call StopVimInTerminal(buf)
+  call delete('XtestPopupLatin')
+  call delete('Xmultibyte')
+endfunc
+
 func Test_popupwin_atcursor_far_right()
   new