Ensure psfc is always allocated

This is needed to avoid a segmentation fault in the call to flux_atmocn
in SMS_Ld2.ne30pg3_t232.BMT1850.derecho_gnu.allactive-defaultio when
running with aoflux_grid=xgrid

mediator/med_phases_aofluxes_mod.F90

@@ -1647,6 +1647,8 @@ subroutine set_aoflux_in_pointers(fldbun_a, fldbun_o, aoflux_in, lsize, xgrid, r
     if (FB_fldchk(fldbun_a, 'Sa_pslv', rc=rc)) then
        call fldbun_getfldptr(fldbun_a, 'Sa_pslv', aoflux_in%psfc, xgrid=xgrid, rc=rc)
        if (chkerr(rc,__LINE__,u_FILE_u)) return
+    else
+       allocate(aoflux_in%psfc(lsize))
     end if
 
     ! if either density or potential temperature are computed, will need bottom level pressure