nwfilter: variable 'obj' must be initialized inside nwfilterBindingCreateXML().
    
    The function nwfilterBindingCreateXML() is failing to compile due to a
    conditional branch which leads to an undefined 'obj' variable. So 'obj'
    must have an initial value to avoid compilation errors. See the problem:
    
      CC       nwfilter/libvirt_driver_nwfilter_impl_la-nwfilter_driver.lo
    nwfilter/nwfilter_driver.c:752:9: error: variable 'obj' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
        if (virNWFilterBindingCreateXMLEnsureACL(conn, def) < 0)
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    nwfilter/nwfilter_driver.c:779:10: note: uninitialized use occurs here
        if (!obj)
             ^~~
    nwfilter/nwfilter_driver.c:752:5: note: remove the 'if' if its condition is always false
        if (virNWFilterBindingCreateXMLEnsureACL(conn, def) < 0)
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    nwfilter/nwfilter_driver.c:742:33: note: initialize the variable 'obj' to silence this warning
        virNWFilterBindingObjPtr obj;
                                    ^
                                     = NULL
    
    This commit initialized 'obj' with NULL to fix the error properly.
    
    Signed-off-by: Julio Faracco <jcfaracco@gmail.com>
    Reviewed-by: Pavel Hrdina <phrdina@redhat.com>