patch 8.0.1346: crash when passing 50 char string to balloon_split()

Problem:    Crash when passing 50 char string to balloon_split().
Solution:   Fix off-by-one error.
diff --git a/src/testdir/test_popup.vim b/src/testdir/test_popup.vim
index 2bb9b32..6250c1b 100644
--- a/src/testdir/test_popup.vim
+++ b/src/testdir/test_popup.vim
@@ -708,6 +708,10 @@
     return
   endif
   call assert_equal([
+        \ 'tempname: 0x555555e380a0 "/home/mool/.viminfz.tmp"',
+        \ ], balloon_split(
+        \ 'tempname: 0x555555e380a0 "/home/mool/.viminfz.tmp"'))
+  call assert_equal([
         \ 'one two three four one two three four one two thre',
         \ 'e four',
         \ ], balloon_split(