asyn-ares: We prefer use of 'CURLcode result'
curl_schannel: Improvements to memory re-allocation strategy

- do not grow memory by doubling its size
- do not leak previously allocated memory if reallocation fails
- replace while-loop with a single check to make sure
  that the requested amount of data fits into the buffer

Bug: http://curl.haxx.se/bug/view.cgi?id=1450
Reported-by: Warren Menzer