fix resource leak in country_graph_coloring_internal

fix resource leak in country_graph_coloring_internal

This addresses the following Coverity warnings:

  Error: CPPCHECK_WARNING (CWE-404): [#def16]
  graphviz-2.40.1/cmd/gvmap/country_graph_coloring.c:275: error[resourceLeak]: Resource leak: fp
  #  273|       }
  #  274|     }
  #  275|-> }
  #  276|
  #  277|   void country_graph_coloring_internal(int seed, SparseMatrix A, int **p, real *norm_1, int do_swapping){

  Error: RESOURCE_LEAK (CWE-772): [#def17]
  graphviz-2.40.1/cmd/gvmap/country_graph_coloring.c:237: alloc_fn: Storage is returned from allocation function "fopen".
  graphviz-2.40.1/cmd/gvmap/country_graph_coloring.c:237: var_assign: Assigning: "fp" = storage returned from "fopen("timing_greedy", "w")".
  graphviz-2.40.1/cmd/gvmap/country_graph_coloring.c:272: noescape: Resource "fp" is not freed or pointed-to in "fprintf". [Note: The source code implementation of the function has been overridden by a builtin model.]
  graphviz-2.40.1/cmd/gvmap/country_graph_coloring.c:275: leaked_storage: Variable "fp" going out of scope leaks the storage it points to.
  #  273|       }
  #  274|     }
  #  275|-> }
  #  276|
  #  277|   void country_graph_coloring_internal(int seed, SparseMatrix A, int **p, real *norm_1, int do_swapping){

Related to #1464.