[gimp] app: in GimpBacktrace Windows backend, return NULL backtrace if not initialized

[Ell <ell src gnome org>]

commit 95b69dd77b172d9f25c4eaae2111b076f14f39c8
Author: Ell <ell_se yahoo com>
Date:   Thu Jul 30 00:13:05 2020 +0300

    app: in GimpBacktrace Windows backend, return NULL backtrace if not initialized
    
    In the Windows implementation of gimp_backtrace_new(), return NULL
    if the backend is not fully initialized.

 app/core/gimpbacktrace-windows.c | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/app/core/gimpbacktrace-windows.c b/app/core/gimpbacktrace-windows.c
index 2355bfe5d1..da1fb734e9 100644
--- a/app/core/gimpbacktrace-windows.c
+++ b/app/core/gimpbacktrace-windows.c
@@ -370,6 +370,9 @@ gimp_backtrace_new (gboolean include_current_thread)
   DWORD          tid;
   gint           i;
 
+  if (! initialized)
+    return NULL;
+
   g_mutex_lock (&mutex);
 
   if (! gimp_backtrace_enumerate_threads ())