bfd: Handle bmmap failure in _bfd_mmap_read_temporary

iovec->bmmap may return MAP_FAILED, which happens in GDB on objects with
iovec == opncls_iovec.  Update _bfd_mmap_read_temporary to handle
iovec->bmmap failure.

	* libbfd.c (_bfd_mmap_read_temporary): Handle iovec->bmmap
	failure.