Check that variables with `intent(out)` are actually set

[zbeekman]

The following procedures have variables with INTENT(OUT) that are not being set:

• Variable bc_conc in subroutine rd_inp_bc_wall in file src/bc/class_bc_wall_procedures.f90
• The entire rd_cgns_mesh procedure if #USE_CGNS is not present in the build
• Variables ov2c_suppl and c2ov_suppl in subroutine supplement_v2c in file src/mesh/tools_mesh.f90
• Variables ov2f_suppl and f2ov_suppl in subroutine supplement_v2f in file src/mesh/tools_mesh.f90
  (Originally submitted by @porteri)