bpo-29941: Assert fixes (python#886)

Make a non-Py_DEBUG, asserts-enabled build of CPython possible. This means
making sure helper functions are defined when NDEBUG is not defined, not
just when Py_DEBUG is defined.

Also fix a division-by-zero in obmalloc.c that went unnoticed because in Py_DEBUG mode, elsize is never zero.

Include/unicodeobject.h

@@ -2313,6 +2313,10 @@ PyAPI_FUNC(Py_UNICODE*) PyUnicode_AsUnicodeCopy(
 PyAPI_FUNC(int) _PyUnicode_CheckConsistency(
     PyObject *op,
     int check_content);
+#elif !defined(NDEBUG)
+/* For asserts that call _PyUnicode_CheckConsistency(), which would
+ * otherwise be a problem when building with asserts but without Py_DEBUG. */
+#define _PyUnicode_CheckConsistency(op, check_content) PyUnicode_Check(op)
 #endif
 
 #ifndef Py_LIMITED_API

Objects/dictobject.c

@@ -437,7 +437,7 @@ static PyObject *empty_values[1] = { NULL };
 /* #define DEBUG_PYDICT */
 
 
-#ifdef Py_DEBUG
+#ifndef NDEBUG
 static int
 _PyDict_CheckConsistency(PyDictObject *mp)
 {

Objects/obmalloc.c

@@ -1227,6 +1227,9 @@ _PyObject_Alloc(int use_calloc, void *ctx, size_t nelem, size_t elsize)
 
     _Py_AllocatedBlocks++;
 
+    if (nelem == 0 || elsize == 0)
+        goto redirect;
+
     assert(nelem <= PY_SSIZE_T_MAX / elsize);
     nbytes = nelem * elsize;
 
@@ -1237,9 +1240,6 @@ _PyObject_Alloc(int use_calloc, void *ctx, size_t nelem, size_t elsize)
         goto redirect;
 #endif
 
-    if (nelem == 0 || elsize == 0)
-        goto redirect;
-
     if ((nbytes - 1) < SMALL_REQUEST_THRESHOLD) {
         LOCK();
         /*

Objects/typeobject.c

@@ -128,7 +128,7 @@ skip_signature(const char *doc)
     return NULL;
 }
 
-#ifdef Py_DEBUG
+#ifndef NDEBUG
 static int
 _PyType_CheckConsistency(PyTypeObject *type)
 {