Added value null check

Author: akarnokd

src/main/java/rx/internal/operators/OperatorOnBackpressureBuffer.java

@@ -126,7 +126,7 @@ public Object peek() {
         @Override
         public Object poll() {
             Object value = queue.poll();
-            if (capacity != null) {
+            if (capacity != null && value != null) {
                 capacity.incrementAndGet();
             }
             return value;