Correct check on availability of dtheta_dt_mix when computing depv_dt_mix

The code to compute depv_dt_mix in the calc_pvBudget routine previously checked
that the dtheta_dt_mp variable had been successfully been retrieved from the
diag pool, when instead the check should have been on the dtheta_dt_mix
variable.

Note that the fix in this commit isn't expected to have any impact in practice,
since both dtheta_dt_mp and dtheta_dt_mix should always be found in the diag
pool.

Author: mgduda

src/core_atmosphere/diagnostics/pv_diagnostics.F

@@ -1457,7 +1457,7 @@ subroutine calc_pvBudget(state, time_lev, diag, mesh, tend, tend_physics)
                depv_dt_mp(k,iCell) = 0.0_RKIND
             end if
 
-            if (associated(dtheta_dt_mp)) then
+            if (associated(dtheta_dt_mix)) then
                call calc_grad_cell(gradtheta, &
                                 iCell, k, nVertLevels, nEdgesOnCell(iCell), verticesOnCell, kiteAreasOnVertex, &
                                 cellsOnCell, edgesOnCell, cellsOnEdge, dvEdge, edgeNormalVectors, &