Fix zlib crash from zlib.decompressobj().flush(val) when val was not positive.

It tried to allocate negative or zero memory.  That fails.

Lib/test/test_zlib.py

@@ -83,6 +83,11 @@ class ExceptionTestCase(unittest.TestCase):
         # verify failure on building decompress object with bad params
         self.assertRaises(ValueError, zlib.decompressobj, 0)
 
+    def test_decompressobj_badflush(self):
+        # verify failure on calling decompressobj.flush with bad params
+        self.assertRaises(ValueError, zlib.decompressobj().flush, 0)
+        self.assertRaises(ValueError, zlib.decompressobj().flush, -1)
+
 
 
 class CompressTestCase(unittest.TestCase):

Modules/zlibmodule.c

@@ -774,6 +774,10 @@ PyZlib_unflush(compobject *self, PyObject *args)
 
     if (!PyArg_ParseTuple(args, "|i:flush", &length))
 	return NULL;
+    if (length <= 0) {
+	PyErr_SetString(PyExc_ValueError, "length must be greater than zero");
+	return NULL;
+    }
     if (!(retval = PyString_FromStringAndSize(NULL, length)))
 	return NULL;