in the frame check to see if content isn't null before trying to recycle it (#519)

Author: robertroeser

rsocket-core/src/main/java/io/rsocket/Frame.java

@@ -159,7 +159,7 @@ public Frame touch(@Nullable Object hint) {
    */
   @Override
   public boolean release() {
-    if (content.release()) {
+    if (content != null && content.release()) {
       recycle();
       return true;
     }
@@ -175,7 +175,7 @@ public boolean release() {
    */
   @Override
   public boolean release(int decrement) {
-    if (content.release(decrement)) {
+    if (content != null && content.release(decrement)) {
       recycle();
       return true;
     }