gdb/

gdb/
	* findvar.c (read_var_value): Never return NULL, throw an error
	instead.  Update the function comment.  State symbol name in the error
	messages.
	* python/py-frame.c (frapy_read_var): Remove handling of NULL from
	read_var_value.
	* stack.c (print_frame_args): Likewise.
	* valops.c (value_of_variable): Likewise.