Bugfix: uninitialized variable debug_ in midas_vertmap

- Reported by @nicjhan in issue #149.
- debug_ was meant to be set to the state of an optional argument
  but if the optional argument was not present debug_ was never set.
  - i_debug and j_debug were also reported as uninitialized but I
    think initializating debug_ will avoid those references.
- No answer changes.

src/initialization/midas_vertmap.F90

@@ -255,20 +255,21 @@ function tracer_z_init(tr_in,z_edges,e,nkml,nkbl,land_fill,wet,nlay,nlevs,debug,
 integer :: k_top,k_bot,k_bot_prev,kk,kstart
 real    :: sl_tr
 real, dimension(size(tr_in,3)) :: wt,z1,z2
-logical :: debug_msg = .false.,debug_=.false.
+logical :: debug_msg, debug_
 
 nx = size(tr_in,1); ny=size(tr_in,2); nz = size(tr_in,3) 
 
 nlevs_data = size(tr_in,3)
-
 if (PRESENT(nlevs)) then
   nlevs_data  = anint(nlevs)
 endif
 
+debug_=.false.
 if (PRESENT(debug)) then
   debug_=debug
 endif
 
+debug_msg = .false.
 if (debug_) then
   debug_msg=.true.
 endif