Fix access violation crash (#2137)

TYPE: bug fix

KEYWORDS: crash, access violation error

SOURCE: Charlie Li, software developer from lakes environmental, Canada

DESCRIPTION OF CHANGES:
Problem:
WRF crashed for access violation frequently, due to the change made in
PR #1991 when namelist.input has sf_urban_physics = 0 and bl_pbl_physics = 1. Because those arrays from urban models are always available, but their memories are not available if the urban option is not turned on. This causes the access violation in module_bl_ysu.F 

Solution:
The fix is actually using v4.5 logic to check on the flag 'flag_bep' before using the arrays.

LIST OF MODIFIED FILES: 
phys/module_bl_ysu.F

TESTS CONDUCTED: 
The Jenkins tests are all passing.

RELEASE NOTE: This PR fixes a access violation error when a PGI compiler is used with urban variables in module_bl_ysu.F when urban option is turned off and the memories of those arrays are not available.

phys/module_bl_ysu.F

@@ -353,27 +353,34 @@ subroutine ysu(u3d,v3d,t3d,qv3d,qc3d,qi3d,p3d,p3di,pi3d,                    &
          present(b_v_bep) .and. present(b_t_bep) .and. present(b_q_bep) .and.  &
          present(b_e_bep) .and. present(dlg_bep) .and. present(dl_u_bep) .and. &
          present(sf_bep)  .and. present(vl_bep)  .and. present(frc_urb2d)) then
-         do k = kts, kte
-            do i = its,ite
-               a_u_hv(i,k)  = a_u_bep(i,k,j)
-               a_v_hv(i,k)  = a_v_bep(i,k,j)
-               a_t_hv(i,k)  = a_t_bep(i,k,j)
-               a_q_hv(i,k)  = a_q_bep(i,k,j)
-               a_e_hv(i,k)  = a_e_bep(i,k,j)
-               b_u_hv(i,k)  = b_u_bep(i,k,j)
-               b_v_hv(i,k)  = b_v_bep(i,k,j)
-               b_t_hv(i,k)  = b_t_bep(i,k,j)
-               b_q_hv(i,k)  = b_q_bep(i,k,j)
-               b_e_hv(i,k)  = b_e_bep(i,k,j)
-               dlg_hv(i,k)  = dlg_bep(i,k,j)
-               dl_u_hv(i,k) = dl_u_bep(i,k,j)
-               vlk_hv(i,k) = vl_bep(i,k,j)
-               sfk_hv(i,k)  = sf_bep(i,k,j)
+
+         ! following v4.5 logic to fix access violation
+         if(flag_bep) then
+
+            do k = kts, kte
+               do i = its,ite
+                  a_u_hv(i,k)  = a_u_bep(i,k,j)
+                  a_v_hv(i,k)  = a_v_bep(i,k,j)
+                  a_t_hv(i,k)  = a_t_bep(i,k,j)
+                  a_q_hv(i,k)  = a_q_bep(i,k,j)
+                  a_e_hv(i,k)  = a_e_bep(i,k,j)
+                  b_u_hv(i,k)  = b_u_bep(i,k,j)
+                  b_v_hv(i,k)  = b_v_bep(i,k,j)
+                  b_t_hv(i,k)  = b_t_bep(i,k,j)
+                  b_q_hv(i,k)  = b_q_bep(i,k,j)
+                  b_e_hv(i,k)  = b_e_bep(i,k,j)
+                  dlg_hv(i,k)  = dlg_bep(i,k,j)
+                  dl_u_hv(i,k) = dl_u_bep(i,k,j)
+                  vlk_hv(i,k) = vl_bep(i,k,j)
+                  sfk_hv(i,k)  = sf_bep(i,k,j)
+               enddo
             enddo
-         enddo
-         do i = its, ite
-            frcurb_hv(i) = frc_urb2d(i,j)
-         enddo
+            do i = its, ite
+               frcurb_hv(i) = frc_urb2d(i,j)
+            enddo
+
+         endif
+
       endif
 
       do i = its, ite