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.