Update AbstractNodeQueue.java to avoid redundant reads of head in peekNode

Avoid spinning on head when a peek/poll observes a null next but the queue is not empty.
does not compile:

[error] /localhome/jenkinsakka/workspace/pr-validator-per-commit-jenkins/akka-actor/src/main/java/akka/dispatch/AbstractNodeQueue.java:64: ';' expected
[error]         return Unsafe.instance.getObjectVolatile(this, tailOffset)) == get();
[error]                                                                   ^
[error] 1 error

Should be:

     return Unsafe.instance.getObjectVolatile(this, tailOffset) == get();