LogServlet: Prevent uncaught exception when param isn't a SHA-1

This is the only instance of (Abbreviated)ObjectId.fromString that is
not properly guarded with isId.

Change-Id: I3c7e44454f476ff1d603552283d0db570f26d080

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

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/LogServlet.java b/gitiles-servlet/src/main/java/com/google/gitiles/LogServlet.java
index 4408c1e..b04540f 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/LogServlet.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/LogServlet.java
@@ -210,7 +210,11 @@
       case 0:
         return Optional.absent();
       case 1:
-        Collection<ObjectId> ids = reader.resolve(AbbreviatedObjectId.fromString(values.get(0)));
+        String id = values.get(0);
+        if (!AbbreviatedObjectId.isId(id)) {
+          return null;
+        }
+        Collection<ObjectId> ids = reader.resolve(AbbreviatedObjectId.fromString(id));
         if (ids.size() != 1) {
           return null;
         }