tests: nsec: correctly free nsec_path

git_buf_clear does not free allocated memory associated with a
git_buf. Use `git_buf_free` instead to correctly free its memory
and plug the memory leak.

tests/index/nsec.c

@@ -56,7 +56,7 @@ static bool should_expect_nsecs(void)
 
 	p_unlink(nsec_path.ptr);
 
-	git_buf_clear(&nsec_path);
+	git_buf_free(&nsec_path);
 
 	return expect;
 }