bpo-29941: Assert fixes (#886) (#955)

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.

(cherry picked from commit a00c3fd12d421e41b769debd7df717d17b0deed5 and 06bb4873d6a9ac303701d08a851d6cd9a51e02a3)
diff --git a/Include/unicodeobject.h b/Include/unicodeobject.h
index 2d0d77e..5b87718 100644
--- a/Include/unicodeobject.h
+++ b/Include/unicodeobject.h
@@ -2309,6 +2309,10 @@
 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
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 8bcc931..566d1a5 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -436,7 +436,7 @@
 /* #define DEBUG_PYDICT */
 
 
-#ifdef Py_DEBUG
+#ifndef NDEBUG
 static int
 _PyDict_CheckConsistency(PyDictObject *mp)
 {
diff --git a/Objects/obmalloc.c b/Objects/obmalloc.c
index a1142f3..32e7ecb 100644
--- a/Objects/obmalloc.c
+++ b/Objects/obmalloc.c
@@ -1227,7 +1227,7 @@
 
     _Py_AllocatedBlocks++;
 
-    assert(nelem <= PY_SSIZE_T_MAX / elsize);
+    assert(elsize == 0 || nelem <= PY_SSIZE_T_MAX / elsize);
     nbytes = nelem * elsize;
 
 #ifdef WITH_VALGRIND