Avoid potential NPE when onError throws. (#4398)

t2 was only initialized if canceling threw an exception. In the happy path it would have never been initialized so when onError threw the call to suppress() would NPE.

Author: JakeWharton

src/main/java/io/reactivex/subscribers/SafeSubscriber.java

@@ -154,8 +154,11 @@ public void onError(Throwable t) {
             }
         } catch (Throwable e) {
             Exceptions.throwIfFatal(e);
-            t2.suppress(e);
-            
+            if (t2 == null) {
+                t2 = new CompositeException(t, e);
+            } else {
+                t2.suppress(e);
+            }
             RxJavaPlugins.onError(t2);
         }
     }