fix resource leakage in gml2gv

fix resource leakage in gml2gv

This addresses the following Coverity warning:

  Error: RESOURCE_LEAK (CWE-772): [#def51]
  graphviz-2.40.1/cmd/tools/gml2gv.c:123: alloc_fn: Storage is returned from allocation function "openFile".
  graphviz-2.40.1/cmd/tools/gml2gv.c:68:5: alloc_fn: Storage is returned from allocation function "fopen".
  graphviz-2.40.1/cmd/tools/gml2gv.c:68:5: var_assign: Assigning: "fp" = "fopen(name, mode)".
  graphviz-2.40.1/cmd/tools/gml2gv.c:79:5: return_alloc: Returning allocated memory "fp".
  graphviz-2.40.1/cmd/tools/gml2gv.c:123: var_assign: Assigning: "outFile" = storage returned from "openFile(optarg, "w")".
  graphviz-2.40.1/cmd/tools/gml2gv.c:123: overwrite_var: Overwriting "outFile" in "outFile = openFile(optarg, "w")" leaks the storage that "outFile" points to.
  #  121|         break;
  #  122|     case 'o':
  #  123|->       outFile = openFile(optarg, "w");
  #  124|         break;
  #  125|     case ':':

Related to #1464.