Fix crash when constructing a QVector with an empty initializer list.

Data::allocate(0) returns a pointer to read-only memory, updating d->size
will segfault. The safety check for this exists in all other QVector ctors
already.

Change-Id: Ida0fe4182de56ee62c7f91e8652cfafbfd7b8410
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>