QCollatorSortKey doesn't work on macOS

QCollatorSortKey doesn't work for one character strings.  Problem is that in QCollator::sortKey() in qcollator_mac.cpp the status returned by UCGetCollationKey is kCollateBufferTooSmall and not kUCOutputBufferTooSmall. This was determined experimentally on a macOS 10.13.3 system. Changing the conditional to call UCGetCollationKey again if the initial buffer is too small to the following seems to fix the problem:

if (status == kUCOutputBufferTooSmall || status == kCollateBufferTooSmall) {