QRingBuffer: optimize memory usage

QByteArray resizing behavior is to preallocate a certain amount
of memory for future growth. Make QRingBuffer aware of the extra
capacity in QByteArray.

Change-Id: I68310d5783fbc32e4fd5075ed3269ce6b7128a92
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qringbuffer_p.h

@@ -113,7 +113,7 @@ public:
 
         // if need buffer reallocation
         if (tail + bytes > buffers.last().size()) {
-            if (tail >= basicBlockSize) {
+            if (tail + bytes > buffers.last().capacity() && tail >= basicBlockSize) {
                 // shrink this buffer to its current size
                 buffers.last().resize(tail);