Initialize variables to avoid the warning message when compiling (#753)

This PR is to initialize variables in observer_fv3reg.f90 to avoid the warning message when compiling.
The change will update regional EnKF excutable. However, the current regression test can't identify the changes in regional EnKF mode. The difference between before and after this update is that no warning message appears when compiling regional EnKF.

This PR is to address #751

**Type of change**
- [x] Bug fix (non-breaking change which fixes an issue)

**Checklist**
- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my own code

src/enkf/observer_fv3reg.f90

@@ -45,7 +45,7 @@ subroutine setup_linhx(rlat, rlon, time, ix, delx, ixp, delxp, iy, dely,  &
   use params, only: nstatefields, nlons, nlats, nlevs, nhr_state, fhr_assim
   use gridinfo, only: npts, latsgrd, lonsgrd
   use statevec, only: nsdim
-  use constants, only: zero,one,pi
+  use constants, only: izero,zero,one,pi
   use mpisetup
   implicit none
 
@@ -54,6 +54,18 @@ subroutine setup_linhx(rlat, rlon, time, ix, delx, ixp, delxp, iy, dely,  &
   real(r_single)                                   ,intent(in   ) :: time         ! observation time relative to middle of window
   integer(i_kind), intent(out) :: ix, iy, it, ixp, iyp, itp
   real(r_kind), intent(out) :: delx, dely, delxp, delyp, delt, deltp
+  ix=izero
+  iy=izero
+  it=izero
+  ixp=izero
+  iyp=izero
+  itp=izero
+  delx=zero
+  dely=zero
+  delxp=zero
+  delyp=zero
+  delt=zero
+  deltp=zero
    write(6,*)'this is a dummy subroutine, running this means something wrong ,stop'
    call stop2(555)
 
@@ -110,6 +122,7 @@ subroutine calc_linhx(hx, dens, dhx_dx, hxpert, hx_ens, &
   type(raggedarr)                                  ,intent(inout) :: hxpert       ! interpolated background
   real(r_single)                                   ,intent(  out) :: hx_ens       ! H (x_ens)
   integer(i_kind) i,j
+  hx_ens=zero
    write(6,*)'this is a dummy subroutine, running this means something wrong ,stop'
    call stop2(555)
 
@@ -145,6 +158,7 @@ subroutine calc_linhx_modens(hx, dhx_dx, hxpert, hx_ens, vscale)
 !$$$
   use kinds, only: r_kind,i_kind,r_single
   use sparsearr, only: sparr, raggedarr
+  use constants, only: zero
   use mpisetup
   implicit none
 
@@ -155,6 +169,7 @@ subroutine calc_linhx_modens(hx, dhx_dx, hxpert, hx_ens, vscale)
   real(r_single)                                   ,intent(  out) :: hx_ens(neigv)! H (x_ens)
   real(r_double),dimension(neigv,nlevs+1)          ,intent(in   ) :: vscale       ! vertical scaling (for modulated ens)
   integer(i_kind) i
+  hx_ens=zero
   write(6,*)'this is a dummy subroutine, running this means something wrong ,stop'
    call stop2(555)