frame_register_unwind(): "frame != NULL" assertion failure

[Andrew Cagney]

> 
> That works.  How does this look?
> 
> 	* frame.c (generic_unwind_get_saved_register): Make non-static.
> 	* frame.h (generic_unwind_get_saved_register): Declare.
> 	* mips-tdep.c (read_next_frame_reg): Fetch register from
> 	current regcache when frame is NULL.
> 	(mips_init_extra_frame_info): Pass NULL explicitly for parameter
> 	that must be NULL.
> 	(mips_get_saved_register): Call generic_unwind_get_saved_register()
> 	instead of frame_register_unwind().
> 
yes, ok.