bpo-36436: Fix _testcapi.pymem_buffer_overflow() (GH-12560)

Handle memory allocation failure. (cherry picked from commit 414b1cd) Co-authored-by: Victor Stinner <vstinner@redhat.com>

Author: miss-islington

Misc/NEWS.d/next/Tests/2019-03-26-13-49-21.bpo-36436.yAtN0V.rst

@@ -0,0 +1 @@
+Fix ``_testcapi.pymem_buffer_overflow()``: handle memory allocation failure.

Modules/_testcapimodule.c

@@ -4175,6 +4175,10 @@ pymem_buffer_overflow(PyObject *self, PyObject *args)
  /* Deliberate buffer overflow to check that PyMem_Free() detects
  the overflow when debug hooks are installed. */
  buffer = PyMem_Malloc(16);
+ if (buffer == NULL) {
+ PyErr_NoMemory();
+ return NULL;
+ }
  buffer[16] = 'x';
  PyMem_Free(buffer);