s390: if -> gdb_assert for tdesc_has_registers check

s390: if -> gdb_assert for tdesc_has_registers check

Before doing the tdesc validation there is a check whether the tdesc has
registers or not.  This check is not only unnecessary but wrong.

First the check is done after a default tdesc is assigned if the original
tdesc has no registers.  These default tdescs always have registers so the
check alway returns true.

Second if the default tdesc would not have registers the check only skips
the tdesc validation instead of returning an error.  This would trigger a
gdb_assert later on in tdesc_use_registers.

gdb/ChangeLog:

	* s390-linux-tdep.c (s390_gdbarch_init): Use gdb_assert for
	tdesc_has_registers check