bfd_error_on_input messages

bfd_error_on_input messages

bfd_errmsg uses asprintf for bfd_error_on_input, which means we
currently leak memory.  Keep a static pointer to the message and free
it in various places to minimise the leaks.
bfd_set_input_error (NULL, bfd_error_no_error) is a way to free up the
last string if that matters.

	* bfd.c (input_error_msg): New static var.
	(bfd_set_input_error): Free it here..
	(bfd_init): ..and here..
	(bfd_errmsg): ..and here.  Use it for asprintf output.