qemu-img: Cannot create fixed vhdx image

When trying to create a fixed vhd image qemu-img will return the
following error:

qemu-img: test.vhdx: Could not create image: Cannot allocate memory

This happens because of a incorrect check in vhdx.c. Specifficaly,
in vhdx_create_bat(), after allocating memory for the BAT entry,
there is a check to determine if the allocation was unsuccsessful.
The error comes from the fact that it checks if s->bat isn't NULL,
which is true in case of succsessful allocation,  and exists with
error ENOMEM.