patch 9.1.0271: CI sound test aborts with undefined variable

Problem:  CI sound test aborts with undefined variable
Solution: initialize g:result in test_sound.vim

closes: #14424

Signed-off-by:
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_sound.vim b/src/testdir/test_sound.vim
index e97ac61..401753e 100644
--- a/src/testdir/test_sound.vim
+++ b/src/testdir/test_sound.vim
@@ -15,6 +15,7 @@
   if has('win32')
     throw 'Skipped: Playing event with callback is not supported on Windows'
   endif
+  let g:result = 0
   let g:playcallback_count = 0
   let g:id = 0
   let event_name = 'bell'
@@ -37,6 +38,7 @@
 func Test_play_silent()
   let fname = fnamemodify('silent.wav', '%p')
   let g:playcallback_count = 0
+  let g:result = -1
 
   " play without callback
   let id1 = sound_playfile(fname)