compose: Hide sending indicator when the message failed validation.

This bug was found when the user tries to send a
over character limit message, i.e It has failed the
validation but it still shows the "Sending..." text.

static/js/compose.js

@@ -284,6 +284,8 @@ export function finish() {
     }
 
     if (!compose_validate.validate()) {
+        // If the message failed validation, hide the sending indicator.
+        $("#sending-indicator").hide();
         return false;
     }