patch 9.0.0864: crash when using "!!" without a previous shell command

Problem:    Crash when using "!!" without a previous shell command.
Solution:   Check "prevcmd" is not NULL. (closes #11487)

src/ex_cmds.c

@@ -858,6 +858,21 @@ free_prev_shellcmd(void)
 }
 #endif
 
+/*
+ * Check that "prevcmd" is not NULL.  If it is NULL then give an error message
+ * and return FALSE.
+ */
+    static int
+prevcmd_is_set(void)
+{
+    if (prevcmd == NULL)
+    {
+	emsg(_(e_no_previous_command));
+	return FALSE;
+    }
+    return TRUE;
+}
+
 /*
  * Handle the ":!cmd" command.	Also for ":r !cmd" and ":w !cmd"
  * Bangs in the argument are replaced with the previously entered command.
@@ -913,9 +928,8 @@ do_bang(
 	    len += (int)STRLEN(newcmd);
 	if (ins_prevcmd)
 	{
-	    if (prevcmd == NULL)
+	    if (!prevcmd_is_set())
 	    {
-		emsg(_(e_no_previous_command));
 		vim_free(newcmd);
 		return;
 	    }
@@ -971,6 +985,9 @@ do_bang(
 
     if (bangredo)	    // put cmd in redo buffer for ! command
     {
+	if (!prevcmd_is_set())
+	    goto theend;
+
 	// If % or # appears in the command, it must have been escaped.
 	// Reescape them, so that redoing them does not substitute them by the
 	// buffername.
@@ -1020,6 +1037,8 @@ do_bang(
 	do_filter(line1, line2, eap, newcmd, do_in, do_out);
 	apply_autocmds(EVENT_SHELLFILTERPOST, NULL, NULL, FALSE, curbuf);
     }
+
+theend:
     if (free_newcmd)
 	vim_free(newcmd);
 }

src/testdir/test_shell.vim

@@ -282,4 +282,17 @@ func Test_shell_repeat()
   let &shell = save_shell
 endfunc
 
+func Test_shell_no_prevcmd()
+  " this doesn't do anything, just check it doesn't crash
+  let after =<< trim END
+    exe "normal !!\<CR>"
+    call writefile([v:errmsg, 'done'], 'Xtestdone')
+    qall!
+  END
+  if RunVim([], after, '--clean')
+    call assert_equal(['E34: No previous command', 'done'], readfile('Xtestdone'))
+  endif
+  call delete('Xtestdone')
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    864,
 /**/
     863,
 /**/