fix definition of GvFILEGV()

fix definition of GvFILEGV()

It was:

    #define GvFILE(gv)   (GvFILE_HEK(gv) ? HEK_KEY(GvFILE_HEK(gv)) : NULL)
    #define GvFILEGV(gv) (gv_fetchfile(GvFILE(gv)))

which is a problem, since gv_fetchfile() doesn't accept a non-null
argument. Change it so that the (cond ? foo : NULL) thing is outside
gv_fetchfile(). This is all a bit academic since GvFILE_HEK should never
be null, but it at least it shuts up a compiler warning.