QArrayData: Don't allocate space we cannot use

When reallocating we assume we will grow more and increase the size to
the next power of 2. However, this might be a size where we end up with
space we cannot take advantage of. Such as having 4 bytes free at the
tail when we have 8 byte objects. The larger the objects are the greater
the chance is that we will end up in this situation, and it would have
a greater chance to leave big allocated chunks unused.

Change-Id: Ifea3d92763c1bafd489b66605c741447e5a57d5a
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qarraydata.cpp

@@ -143,7 +143,7 @@ qCalculateGrowingBlockSize(size_t elementCount, size_t elementSize, size_t heade
     }
 
     result.elementCount = (bytes - unsigned(headerSize)) / unsigned(elementSize);
-    result.size = bytes;
+    result.size = result.elementCount * elementSize + headerSize;
     return result;
 }