BufferedIO: fix bug in writeBufNonBlocking introduced by commit 7d39e100
bufferAdjustL does nothing but update bufL, and set bufL = bufR = 0 when bufL == bufR. It ignores the old bufL. writeBufNonBlocking is currently unused. It is only used to implement flushWriteBuffer0, which is never called by base (hPutBufNonBlocking uses a blocking flush).