Fix NPE introduced by injars/rules fixed point.

Change-Id: I002cb94c8df07eafd95f34d40d0106d21e067fe8
diff --git a/src/main/java/com/android/tools/r8/R8Command.java b/src/main/java/com/android/tools/r8/R8Command.java
index d017a2f..9f5c949 100644
--- a/src/main/java/com/android/tools/r8/R8Command.java
+++ b/src/main/java/com/android/tools/r8/R8Command.java
@@ -692,7 +692,9 @@
         for (FilteredClassPath injar : parser.getConfigurationBuilder().getInjars()) {
           if (seenInjars.add(injar)) {
             ArchiveResourceProvider provider = getAppBuilder().createAndAddProvider(injar);
-            providers.add(provider);
+            if (provider != null) {
+              providers.add(provider);
+            }
           }
         }
         if (providers.isEmpty()) {