c++ frontend: check for missing condition for novector [PR115623]

c++ frontend: check for missing condition for novector [PR115623]

It looks like I forgot to check in the C++ frontend if a condition exist for the
loop being adorned with novector.  This causes a segfault because cond isn't
expected to be null.

This fixes it by issuing ignoring the pragma when there's no loop condition
the same way we do in the C frontend.

gcc/cp/ChangeLog:

	PR c++/115623
	* semantics.cc (finish_for_cond): Add check for C++ cond.

gcc/testsuite/ChangeLog:

	PR c++/115623
	* g++.dg/vect/vect-novector-pragma_2.cc: New test.