Fix access violation when disassembling a corrupt VMS binary.

Fix access violation when disassembling a corrupt VMS binary.

	PR 21615
	* vms-alpha.c (_bfd_vms_slurp_egsd): Use unsigned int for
	gsd_size.  Check that there are enough bytes remaining to read the
	type and size of the next egsd.  Check that the size of the egsd
	does not exceed the size of the record.