patch 8.0.1202: :wall gives an errof for a terminal window

Problem:    :wall gives an errof for a terminal window. (Marius Gedminas)
Solution:   Don't try writing a buffer that can't be written. (Yasuhiro
            Matsumoto, closes #2190)
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index b4351b2..60f47bb 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -3400,7 +3400,7 @@
 
     FOR_ALL_BUFFERS(buf)
     {
-	if (bufIsChanged(buf))
+	if (bufIsChanged(buf) && !bt_dontwrite(buf))
 	{
 	    /*
 	     * Check if there is a reason the buffer cannot be written: