closes bpo-41533: Fix a potential memory leak when allocating a stack (pythonGH-21847)

Free the stack allocated in va_build_stack if do_mkstack fails
and the stack is not a small_stack

Author: tontinton

Misc/NEWS.d/next/Core and Builtins/2020-08-12-20-29-57.bpo-41533.4pcVAc.rst

@@ -0,0 +1,2 @@
+Free the stack allocated in ``va_build_stack`` if ``do_mkstack`` fails and
+the stack is not a ``small_stack``.

Python/modsupport.c

@@ -622,6 +622,9 @@ va_build_stack(PyObject **small_stack, Py_ssize_t small_stack_len,
     va_end(lva);
 
     if (res < 0) {
+        if (stack != small_stack) {
+            PyMem_Free(stack);
+        }
         return NULL;
     }