Bugfix: uninitialized variable dS in midas_vertmap

- Reported by @nicjhan in issue mom-ocean#149.
- A maxval(abs(dS)) is using all of the array and yet only
  a sub-array is meant to be used. I simply initialized dS(:,:)=0
- No answer changes.

src/initialization/midas_vertmap.F90

@@ -534,10 +534,10 @@ subroutine determine_temperature(temp,salt,R,p_ref,niter,land_fill,h,k_start,eos
 
 nx=size(temp,1);ny=size(temp,2); nz=size(temp,3)
 
-
 press(:) = p_ref
 
 do j=1,ny
+  dS(:,:) = 0. ! Needs to be zero everywhere since there is a maxval(abs(dS)) later...
   T=temp(:,j,:)
   S=salt(:,j,:)
   hin=h(:,j,:)