c++: Fix a compile time memory leak in finish_static_assert

c++: Fix a compile time memory leak in finish_static_assert

On Tue, Nov 28, 2023 at 11:31:48AM -0500, Jason Merrill wrote:
> Jonathan pointed out elsewhere that this gets leaked if error return
> prevents us from getting to the XDELETEVEC.

As there is a single error return in which it can leak, I've just added
a XDELETEVEC (buf); statement to that path rather than introducing some
RAII solution.

2023-11-29  Jakub Jelinek  <jakub@redhat.com>

	* semantics.cc (finish_static_assert): Free buf on error return.