HostIndexServlet: Prevent StringIndexOutOfBoundsException

Bug: Issue 91
Change-Id: Id4c4f9f5cf53321ec97ba355508264c929b9fe2f

gitiles-servlet/src/main/java/com/google/gitiles/HostIndexServlet.java

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/HostIndexServlet.java b/gitiles-servlet/src/main/java/com/google/gitiles/HostIndexServlet.java
index d8aec26..79d8927 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/HostIndexServlet.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/HostIndexServlet.java
@@ -114,7 +114,9 @@
 
     SoyListData repos = new SoyListData();
     for (RepositoryDescription desc : descs.values()) {
-      repos.add(toSoyMapData(desc, prefix, view));
+      if (prefix == null || desc.name.startsWith(prefix)) {
+        repos.add(toSoyMapData(desc, prefix, view));
+      }
     }
 
     String hostName = urls.getHostName(req);
@@ -175,7 +177,10 @@
   }
 
   private static String stripPrefix(@Nullable String prefix, String name) {
-    return prefix != null ? name.substring(prefix.length() + 1) : name;
+    if (prefix != null && name.startsWith(prefix)) {
+      return name.substring(prefix.length() + 1);
+    }
+    return name;
   }
 
   private static Set<String> parseShowBranch(HttpServletRequest req) {