Reinitialize fire btran2 to spval for all patches in each time step

This, or something like it, is important so that the fire code isn't
trying to use btran2 from earlier time steps for patches that were in
the exposed veg filter at one point but no longer are. The fix here is
not the cleanest way to fix this issue, but represents a minimal set of
changes to fix the issue. In a later commit, I'll try to fix this in a
better way (which should be bit-for-bit with this fix).

Resolves #1153 (Fire btran2 is only computed for exposed veg
patches, but is used over all veg patches)

src/biogeochem/CNFireBaseMod.F90

@@ -201,6 +201,8 @@ subroutine CNFire_calc_fire_root_wetness( this, bounds, num_exposedvegp, filter_
     !
     ! Calculate the root wetness term that will be used by the fire model
     !
+    ! FIXME(wjs, 2020-10-02) remove next line
+    use clm_varcon      , only : spval
     use pftconMod                 , only : pftcon
     use clm_varpar                , only : nlevgrnd
     use PatchType                 , only : patch
@@ -232,6 +234,9 @@ subroutine CNFire_calc_fire_root_wetness( this, bounds, num_exposedvegp, filter_
          h2osoi_vol    => waterstatebulk_inst%h2osoi_vol_col  & ! Input:  [real(r8) (:,:) ]  volumetric soil water (0<=h2osoi_vol<=watsat) [m3/m3] (porosity)   (constant)
          )
 
+    ! FIXME(wjs, 2020-10-02) Remove next line
+    btran2(bounds%begp:bounds%endp) = spval
+
       do f = 1, num_exposedvegp
          p = filter_exposedvegp(f)
          btran2(p)   = btran0

src/biogeochem/CNFireLi2021Mod.F90

@@ -679,6 +679,8 @@ subroutine CNFire_calc_fire_root_wetness( this, bounds, num_exposedvegp, filter_
     !
     ! Calculate the root wetness term that will be used by the fire model
     !
+    ! FIXME(wjs, 2020-10-02) remove next line
+    use clm_varcon      , only : spval
     use pftconMod                 , only : pftcon
     use PatchType                 , only : patch
     class(cnfire_li2021_type) :: this
@@ -702,6 +704,9 @@ subroutine CNFire_calc_fire_root_wetness( this, bounds, num_exposedvegp, filter_
          h2osoi_vol    => waterstatebulk_inst%h2osoi_vol_col  & ! Input:  [real(r8) (:,:) ]  volumetric soil water (0<=h2osoi_vol<=watsat) [m3/m3] (porosity)   (constant)
          )
 
+    ! FIXME(wjs, 2020-10-02) Remove next line
+    btran2(bounds%begp:bounds%endp) = spval
+
       do f = 1, num_exposedvegp
          p = filter_exposedvegp(f)
          btran2(p)   = 0._r8