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

Handle memory allocation failure.

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

@@ -4184,6 +4184,10 @@ 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);