fix resource leakage in gvpack

fix resource leakage in gvpack

This addresses the following Coverity warning:

  Error: RESOURCE_LEAK (CWE-772): [#def60]
  graphviz-2.40.1/cmd/tools/gvpack.c:239: alloc_fn: Storage is returned from allocation function "openFile".
  graphviz-2.40.1/cmd/tools/gvpack.c:124:5: alloc_fn: Storage is returned from allocation function "fopen".
  graphviz-2.40.1/cmd/tools/gvpack.c:124:5: var_assign: Assigning: "fp" = "fopen(name, mode)".
  graphviz-2.40.1/cmd/tools/gvpack.c:134:5: return_alloc: Returning allocated memory "fp".
  graphviz-2.40.1/cmd/tools/gvpack.c:239: var_assign: Assigning: "outfp" = storage returned from "openFile(optarg, "w")".
  graphviz-2.40.1/cmd/tools/gvpack.c:239: overwrite_var: Overwriting "outfp" in "outfp = openFile(optarg, "w")" leaks the storage that "outfp" points to.
  #  237|         break;
  #  238|     case 'o':
  #  239|->       outfp = openFile(optarg, "w");
  #  240|         break;
  #  241|     case 'u':

Related to #1464.