S390: Fix crash when remote tdesc doesn't define vec128

S390: Fix crash when remote tdesc doesn't define vec128

I've encountered a GDB crash when trying to read registers from a remote
stub that provided a target.xml with vector registers, but without the
'vec128' data type.  The crash is caused by NULL register type entries for
the "concatenated" pseudo-registers v0-v15.  These NULL entries are
introduced by the logic in s390_pseudo_register_type(), where the tdesc
type 'vec128' is returned unconditionally -- even if it doesn't exist (is
NULL).

The fixed logic for determining a "concatenated" vector register's type
now returns the type of the raw register v16 instead.  This also makes
sure that all vector register have the same type.

gdb/ChangeLog:

	* s390-tdep.c (s390_pseudo_register_type): For v0-v15 don't yield
	the possibly non-existent tdesc type 'vec128', but the type of raw
	register v16 instead.