Config file open: don't free memory that doesn't belong to us

On error, it would free the configuration object even though it didn't
own that memory, which would cause a double-free.

This fixes the first part of Issue #210

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>

src/config_file.c

@@ -261,7 +261,6 @@ static int config_open(git_config_file *cfg)
  cleanup:
 	cvar_list_free(&b->var_list);
 	gitfo_free_buf(&b->reader.buffer);
-	free(cfg);
 
 	return git__rethrow(error, "Failed to open config");
 }