ice_grid: initialize 'l_readCenter' for all grid types (#758)

In 3fedc78 (Allow for read of tlat, tlon, anglet with popgrid (#463),
2020-06-24), ice_grid::init_grid2 was changed so that ice_grid::Tlatlon,
which computes the TLAT and TLON arrays from ULAT and ULON, is only
called if the private module variable 'l_readCenter' is false.

The idea is that if the grid file contains a variable 'anglet', then it
is assumed that it also contains variables 'tlon' and 'tlat', and so
these fields are read directly instead of being computed.

This logic, however, was only implemented in ice_grid::popgrid_nc, which
sets 'l_readCenter' depending on the presence or absence of 'anglet' in
the grid file. This means that if 'popgrid_nc' is not called (for
example with "grid_format='bin'", in which case init_grid2 calls
'popgrid' and not 'popgrid_nc'), then 'l_readCenter' is used
uninitialized in init_grid2, and so it's possible that 'Tlatlon' is not
called, if 'l_readCenter' happens to be initialized to true.

This in turns leads to 'TLAT' and 'TLON' being uninitialized, and the
code failing when accessing these arrays if compiling with NaN
initialization.

Fix this by initializing 'l_readCenter' at the beginning of init_grid2,
such that it is initialized for all choices of 'grid_format' and
'grid_type'. Remove the initialization in 'popgrid_nc'.

cicecore/cicedynB/infrastructure/ice_grid.F90

@@ -485,6 +485,8 @@ subroutine init_grid2
       ! lat, lon, cell widths, angle, land mask
       !-----------------------------------------------------------------
 
+      l_readCenter = .false.
+
       call icepack_query_parameters(pi_out=pi, pi2_out=pi2, puny_out=puny)
       call icepack_warnings_flush(nu_diag)
       if (icepack_warnings_aborted()) call abort_ice(error_message=subname, &
@@ -955,7 +957,6 @@ subroutine popgrid_nc
       call ice_open_nc(kmt_file,fid_kmt)
 
       diag = .true.       ! write diagnostic info
-      l_readCenter = .false.
       !-----------------------------------------------------------------
       ! topography
       !-----------------------------------------------------------------