Ticket #4213: workaround to avoid crash on FreeBSD.

In the read_subshell_prompt() (src/subshell/common.c:1546) the first
call of select() returns 0. Therefore subshell_prompt is NULL.
NULL dereference is the cause of crash.

This commit isn't a real fix but just a workaround.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>

src/filemanager/layout.c

@@ -978,6 +978,10 @@ setup_cmdline (void)
         return;
 
 #ifdef ENABLE_SUBSHELL
+    /* Workaround: avoid crash on FreeBSD (see ticket #4213 for details)  */
+    if (subshell_prompt == NULL)
+        return;
+
     if (mc_global.tty.use_subshell)
     {
         tmp_prompt = g_string_free (subshell_prompt, FALSE);