Fix SF #544995 (zlib crash on second flush call)

Bug fix by mhammond.

Bug fix candidate for 2.2, not present in 2.1.

Lib/test/test_zlib.py

@@ -41,6 +41,12 @@ buf = buf * 16
 co = zlib.compressobj(8, 8, -15)
 x1 = co.compress(buf)
 x2 = co.flush()
+try:
+    co.flush()
+    print "Oops - second flush worked when it should not have!"
+except zlib.error:
+    pass
+
 x = x1 + x2
 
 dc = zlib.decompressobj(-15)

Modules/zlibmodule.c

@@ -648,6 +648,7 @@ PyZlib_flush(compobject *self, PyObject *args)
 	zlib_error(self->zst, err, "while flushing");
 	Py_DECREF(RetVal);
 	RetVal = NULL;
+	goto error;
     }
 
     if (_PyString_Resize(&RetVal, self->zst.total_out - start_total_out) < 0)