RFA: PATCH to check for overflow in make_tree_vec_stat

[Jason Merrill]

A patch I've been putting together ran into strange memory corruption
issues which turned out to be because the calculation in
make_tree_vec_stat was overflowing and allocating a small TREE_VEC
instead of a large one.  This assert should work as a simple sanity
check.

Tested x86_64-pc-linux-gnu, OK for trunk?

commit 59ccf3b1dd5aaf9611a133ad55d950de525e862d
Author: Jason Merrill <ja...@redhat.com>
Date:   Thu May 18 15:23:53 2017 -0400

            * tree.c (make_tree_vec_stat): Check for overflow.

diff --git a/gcc/tree.c b/gcc/tree.c
index 7506725..327332b 100644
--- a/gcc/tree.c
+++ b/gcc/tree.c
@@ -2270,6 +2270,9 @@ make_tree_vec_stat (int len MEM_STAT_DECL)
   tree t;
   int length = (len - 1) * sizeof (tree) + sizeof (struct tree_vec);
 
+  /* Cheap check for overflow.  */
+  gcc_assert (length > len);
+
   record_node_allocation_statistics (TREE_VEC, length);
 
   t = ggc_alloc_cleared_tree_node_stat (length PASS_MEM_STAT);