don't access cfun in dump_function_to_file

don't access cfun in dump_function_to_file

dump_function_to_file takes the function to dump as a parameter, and
parts of it use the local fun variable where cfun would be used
elsewhere.  Others use cfun, presumably in error.  Fixed to use fun
uniformly.  Added a few more tests for non-NULL fun before
dereferencing it.

for  gcc/ChangeLog

	* tree-cfg.c (dump_function_to_file): Use fun, not cfun.