powerpc/powernv/sriov: perform null check on iov before dereferencing iov

In the Linux kernel, the following vulnerability has been resolved:

powerpc/powernv/sriov: perform null check on iov before dereferencing iov

Currently pointer iov is being dereferenced before the null check of iov

which can lead to null pointer dereference errors. Fix this by moving the

iov null check before the dereferencing.

Detected using cppcheck static analysis:

linux/arch/powerpc/platforms/powernv/pci-sriov.c:597:12: warning: Either

the condition '!iov' is redundant or there is possible null pointer

dereference: iov. [nullPointerRedundantCheck]

num_vfs = iov->num_vfs;

^