show warning when opening invalid jid

src/main/java/eu/siacs/conversations/ui/StartConversationActivity.java

@@ -1193,6 +1193,10 @@ public class StartConversationActivity extends XmppActivity implements OnRosterU
         public String account;
 
         boolean invite() {
+            if (!isJidValid()) {
+                Toast.makeText(StartConversationActivity.this,R.string.invalid_jid,Toast.LENGTH_SHORT).show();
+                return false;
+            }
             if (getJid() != null) {
                 return handleJid(this);
             }

src/main/java/eu/siacs/conversations/utils/XmppUri.java

@@ -151,6 +151,15 @@ public class XmppUri {
 		}
 	}
 
+	public boolean isJidValid() {
+		try {
+			Jid.fromString(jid);
+			return true;
+		} catch (InvalidJidException e) {
+			return false;
+		}
+	}
+
 	public String getBody() {
 		return body;
 	}