qemu: Avoid NULL dereference in qemuSharedDiskEntryFree
qemu: Avoid NULL dereference in qemuSharedDiskEntryFree

At least one caller may call qemuSharedDiskEntryFree with NULL as the
first argument. Let's make the function similar to other *Free functions
and do nothing in such case.

shugaoye/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,libvirt/libvirt,jardasgit/libvirt,jfehlig/libvirt,zhlcindy/libvirt-1.1.4-maintain,crobinso/libvirt,agx/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,olafhering/libvirt,nertpinx/libvirt,libvirt/libvirt,nertpinx/libvirt,crobinso/libvirt,nertpinx/libvirt,rlaager/libvirt,jfehlig/libvirt,jardasgit/libvirt,crobinso/libvirt,rlaager/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,elmarco/libvirt,andreabolognani/libvirt,libvirt/libvirt,shugaoye/libvirt,taget/libvirt,cbosdo/libvirt,zippy2/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,elmarco/libvirt,cbosdo/libvirt,nertpinx/libvirt,eskultety/libvirt,libvirt/libvirt,rlaager/libvirt,agx/libvirt,elmarco/libvirt,datto/libvirt,andreabolognani/libvirt,shugaoye/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,agx/libvirt,jfehlig/libvirt,taget/libvirt,jardasgit/libvirt,VenkatDatta/libvirt,datto/libvirt,datto/libvirt,olafhering/libvirt,jardasgit/libvirt,agx/libvirt,shugaoye/libvirt,agx/libvirt,rlaager/libvirt,rlaager/libvirt,nertpinx/libvirt,datto/libvirt,crobinso/libvirt,eskultety/libvirt,taget/libvirt,cbosdo/libvirt,VenkatDatta/libvirt,eskultety/libvirt,zhlcindy/libvirt-1.1.4-maintain,taget/libvirt,zippy2/libvirt,zippy2/libvirt,shugaoye/libvirt,andreabolognani/libvirt,elmarco/libvirt,zhlcindy/libvirt-1.1.4-maintain,elmarco/libvirt,VenkatDatta/libvirt,eskultety/libvirt,zhlcindy/libvirt-1.1.4-maintain,cbosdo/libvirt,olafhering/libvirt,eskultety/libvirt,datto/libvirt,jardasgit/libvirt,zhlcindy/libvirt-1.1.4-maintain,zippy2/libvirt,olafhering/libvirt,cbosdo/libvirt,taget/libvirt