BUG/MINOR: h2: do not accept SETTINGS_ENABLE_PUSH other than 0 or 1

We don't use yet it but for correctness, let's enforce the check.

To backport to 1.8.

src/mux_h2.c

@@ -1102,6 +1102,12 @@ static int h2c_handle_settings(struct h2c *h2c)
 			}
 			h2c->mfs = arg;
 			break;
+		case H2_SETTINGS_ENABLE_PUSH:
+			if (arg < 0 || arg > 1) { // RFC7540#6.5.2
+				error = H2_ERR_PROTOCOL_ERROR;
+				goto fail;
+			}
+			break;
 		}
 	}