Fix NPE in AbstractReturnValueIgnored for lambdas.

Fixes #657

RELNOTES: Fixes a null pointer exception in AbstractReturnValueIgnored on expression-type lambdas.

MOE_MIGRATED_REVID=160684735

check_api/src/main/java/com/google/errorprone/matchers/IsLastStatementInBlock.java

@@ -28,6 +28,6 @@ public class IsLastStatementInBlock<T extends StatementTree> implements Matcher<
   public boolean matches(T statement, VisitorState state) {
     BlockTree block = state.findEnclosing(BlockTree.class);
 
-    return Iterables.getLast(block.getStatements()).equals(statement);
+    return block != null && Iterables.getLast(block.getStatements()).equals(statement);
   }
 }

core/src/test/java/com/google/errorprone/bugpatterns/testdata/ReturnValueIgnoredPositiveCases.java

@@ -27,6 +27,9 @@
 public class ReturnValueIgnoredPositiveCases {
   String a = "thing";
 
+  // BUG: Diagnostic contains: Return value of this method must be used
+  private Runnable r = () -> String.valueOf("");
+
   { // String methods
     // BUG: Diagnostic contains: remove this line
     String.format("%d", 10);