alpha-vms: large memory allocation

This patch simplifies reading of image headers.  It's really not worth
trying to avoid re-reading a 12 byte buffer and then read in
VMS_BLOCK_SIZE chunks, better just to throw the buffer away and use
_bfd_malloc_and_read which does checks against file size.

	* vms-alpha.c (alpha_vms_object_p): Use _bfd_malloc_and_read.
	Remove duplicate undersize check.