storage: Fix coverity warning
storage: Fix coverity warning

Introduced by commit e0139e30444:

1777 	    /* Updating pool metadata */

(40) Event var_deref_op: Dereferencing null pointer "newvol".
     Also see events: [assign_zero]

1778 	    pool->def->allocation += newvol->allocation;
1779 	    pool->def->available -= newvol->allocation;

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