Avoid possible command injection

Because we are running cmd.exe here, it is possible to use a malicious URL to execute arbitrary commands on the shell ( e.g. if url=http://my-patch-notes.com/" & del C:\ )

(Triggered by internal security audit and Fortify analysis.)

core/src/main/java/com/threerings/getdown/util/StringUtil.java

@@ -9,6 +9,14 @@ import java.util.StringTokenizer;
 
 public class StringUtil {
 
+    /**
+     * @return true if the specified string could be a valid URL (contains no illegal characters)
+     */
+    public static boolean couldBeValidUrl (String url)
+    {
+        return url.matches("[A-Za-z0-9\\-\\._~:/\\?#\\[\\]@!$&'\\(\\)\\*\\+,;=%]+");
+    }
+
     /**
      * @return true if the string is null or consists only of whitespace, false otherwise.
      */

core/src/test/java/com/threerings/getdown/util/StringUtilTest.java

@@ -0,0 +1,28 @@
+//
+// Getdown - application installer, patcher and launcher
+// Copyright (C) 2004-2016 Getdown authors
+// https://github.com/threerings/getdown/blob/master/LICENSE
+
+package com.threerings.getdown.util;
+
+import org.junit.Test;
+
+import static com.threerings.getdown.util.StringUtil.couldBeValidUrl;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+/**
+ * Tests {@link StringUtil}.
+ */
+public class StringUtilTest
+{
+    @Test public void testCouldBeValidUrl ()
+    {
+        assertTrue(couldBeValidUrl("http://www.foo.com/"));
+        assertTrue(couldBeValidUrl("http://www.foo.com/A-B-C/1_2_3/~bar/q.jsp?x=u+i&y=2;3;4#baz%20baz"));
+        assertTrue(couldBeValidUrl("https://user:secret@www.foo.com/"));
+
+        assertFalse(couldBeValidUrl("http://www.foo.com & echo hello"));
+        assertFalse(couldBeValidUrl("http://www.foo.com\""));
+    }
+}