bpo-34408: Prevent a null pointer dereference and resource leakage in `PyInterpreterState_New()` (GH-8767)

* A pointer in `PyInterpreterState_New()` could have been `NULL` when being dereferenced.

* Memory was leaked in `PyInterpreterState_New()` when taking some error-handling code path.

Misc/NEWS.d/next/Core and Builtins/2018-08-14-22-35-19.bpo-34408.aomWYW.rst

@@ -0,0 +1 @@
+Prevent a null pointer dereference and resource leakage in ``PyInterpreterState_New()``.

Python/pystate.c

@@ -172,23 +172,27 @@ PyInterpreterState_New(void)
     interp->pyexitmodule = NULL;
 
     HEAD_LOCK();
-    interp->next = _PyRuntime.interpreters.head;
-    if (_PyRuntime.interpreters.main == NULL) {
-        _PyRuntime.interpreters.main = interp;
-    }
-    _PyRuntime.interpreters.head = interp;
     if (_PyRuntime.interpreters.next_id < 0) {
         /* overflow or Py_Initialize() not called! */
         PyErr_SetString(PyExc_RuntimeError,
                         "failed to get an interpreter ID");
-        /* XXX deallocate! */
+        PyMem_RawFree(interp);
         interp = NULL;
     } else {
         interp->id = _PyRuntime.interpreters.next_id;
         _PyRuntime.interpreters.next_id += 1;
+        interp->next = _PyRuntime.interpreters.head;
+        if (_PyRuntime.interpreters.main == NULL) {
+            _PyRuntime.interpreters.main = interp;
+        }
+        _PyRuntime.interpreters.head = interp;
     }
     HEAD_UNLOCK();
 
+    if (interp == NULL) {
+        return NULL;
+    }
+
     interp->tstate_next_unique_id = 0;
 
     return interp;