QRingBuffer: avoid allocation in c'tor

Reduces creation time and memory consumption when the buffer is unused.
Robin's benchmark:

  QBENCHMARK {
      QFile file(filename);
  }

reports the following results (run with -median 10 -iterations 524288):

before: 0.00028 msecs per iteration (total: 149, iterations: 524288)
after:  0.00017 msecs per iteration (total: 93, iterations: 524288).

Change-Id: Ied4e7caeca794b94260b8fc59b3ba656f4719c30
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

0 files changed, 0 insertions, 0 deletions