Bugfix: check symbol for NULL in binding json dump

When dumping a binding, as is done during the binding event, check
symbol for NULL. If it is, dump json null. This prevents a crash when
running a binding that was configured with bindcode.

fixes #1379

docs/ipc

@@ -769,9 +769,9 @@ input_code (integer)::
 	If the binding was configured with +bindcode+, this will be the key code
 	that was given for the binding. If the binding is a mouse binding, it will be
 	the number of the mouse button that was pressed. Otherwise it will be 0.
-symbol (string)::
+symbol (string or null)::
 	If this is a keyboard binding that was configured with +bindsym+, this
-	field will contain the given symbol.
+	field will contain the given symbol. Otherwise it will be +null+.
 input_type (string)::
 	This will be +"keyboard"+ or +"mouse"+ depending on whether or not this was
 	a keyboard or a mouse binding.

src/ipc.c

@@ -160,6 +160,9 @@ static void dump_binding(yajl_gen gen, Binding *bind) {
     ystr((const char*)(bind->input_type == B_KEYBOARD ? "keyboard" : "mouse"));
 
     ystr("symbol");
+    if (bind->symbol == NULL)
+        y(null);
+    else
         ystr(bind->symbol);
 
     ystr("command");