Fix for a do loop index in bep_bem urban code (wrf-model#2196)

TYPE: bug fix

KEYWORDS: module_sf_bep_bem.F, loop index, debugging-crash

SOURCE: L. Fita (UBA/CIMA/IFAECI)

DESCRIPTION OF CHANGES:
Problem:
The loop index in line 1600 of subroutine module_sf_bep_bem.F should be up to `nzu`, instead of `nz_um`.

Solution:
This PR fixes the index.

ISSUE: For use when this PR closes an issue.
Fixes wrf-model#2194

LIST OF MODIFIED FILES: 
M       phys/module_sf_bep_bem.F

TESTS CONDUCTED: 
1. Code was modified and recompiled in serial debug mode and it did not crashed
2. The Jenkins tests are all passing.

RELEASE NOTE: This PR fixed a loop index error in bep_bem urban code.

Author: LluisFB

phys/module_sf_bep_bem.F

@@ -1597,7 +1597,7 @@ subroutine BEP1D(itimestep,ix,iy,iurb,kms,kme,kts,kte,z,dt,ua,va,pt,da,pr,pt0,
 
 
 
-       do iz=1,nz_um !Compute the outdoor temperature 
+       do iz=1,nzu !Compute the outdoor temperature 
 	 tmp_u(iz)=pt_u(iz)*(pr_u(iz)/p0)**(rcp_u) 
        end do