HybridLocalStrategy: NPE fix while canceling delegating job

Summary: ^

Reviewed By: cjhopman

fbshipit-source-id: c5a8f0b81835e80e680ba1cb4b0a8c7b291928f8

Author: mykola-semko

src/com/facebook/buck/rules/modern/builders/HybridLocalStrategy.java

@@ -278,6 +278,10 @@ public ListenableFuture<?> rescheduleLocally(Throwable reason) {
 
     public boolean cancelDelegateLocked(Throwable reason) {
       Verify.verify(Thread.holdsLock(this));
+      if (delegateResult == null) {
+        LOG.debug("Nothing to cancel.");
+        return false;
+      }
 
       StrategyBuildResult capturedDelegateResult = Objects.requireNonNull(delegateResult);
       if (capturedDelegateResult.getBuildResult().isDone()) {