bpo-1621: Avoid signed integer overflow in set_table_resize(). (GH-9059)

Address a C undefined behavior signed integer overflow issue in set object table resizing.  Our -fwrapv compiler flag and practical reasons why sets are unlikely to get this large should mean this was never an issue but it was incorrect code that generates code analysis warnings.

<!-- issue-number: [bpo-1621](https://www.bugs.python.org/issue1621) -->
https://bugs.python.org/issue1621
<!-- /issue-number -->

Author: sir-sigurd

Misc/NEWS.d/next/Core and Builtins/2018-09-11-15-19-37.bpo-1621.7o19yG.rst

@@ -0,0 +1,2 @@
+Do not assume signed integer overflow behavior (C undefined behavior) when
+performing set hash table resizing.

Objects/setobject.c

@@ -302,7 +302,6 @@ actually be smaller than the old one.
 static int
 set_table_resize(PySetObject *so, Py_ssize_t minused)
 {
-    Py_ssize_t newsize;
     setentry *oldtable, *newtable, *entry;
     Py_ssize_t oldmask = so->mask;
     size_t newmask;
@@ -313,13 +312,9 @@ set_table_resize(PySetObject *so, Py_ssize_t minused)
 
     /* Find the smallest table size > minused. */
     /* XXX speed-up with intrinsics */
-    for (newsize = PySet_MINSIZE;
-         newsize <= minused && newsize > 0;
-         newsize <<= 1)
-        ;
-    if (newsize <= 0) {
-        PyErr_NoMemory();
-        return -1;
+    size_t newsize = PySet_MINSIZE;
+    while (newsize <= (size_t)minused) {
+        newsize <<= 1; // The largest possible value is PY_SSIZE_T_MAX + 1.
     }
 
     /* Get space for a new table. */