Make rehash(0) work with ubsan's unsigned-integer-overflow.

Reviewers: mclow.lists, EricWF

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D40743

include/__hash_table

@@ -2137,7 +2137,7 @@ template <class _Tp, class _Hash, class _Equal, class _Alloc>
 void
 __hash_table<_Tp, _Hash, _Equal, _Alloc>::rehash(size_type __n)
 {
-    if (__n == 1)
+    if (__n < 2)
         __n = 2;
     else if (__n & (__n - 1))
         __n = __next_prime(__n);