RFR [11] 8207960: Non-negative WINDOW_UPDATE increments may leave the stream window size negative

[Chris Hegarty]

The stream window size can correctly become negative after processing
the initial SETTINGS_INITIAL_WINDOW_SIZE. Stream specific
WINDOW_UPDATE's should not cause a stream reset if the current
window size is negative before the positive increment amount is added. 

http://cr.openjdk.java.net/~chegar/8207960/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8207960

-Chris.