Fix Fatal Python error: UNREF invalid object in debug builds

PyObject_Del() should only be called on the self object in
a dealloc call, not after failing to make a new object.
Replace with Py_DECREF, which eventually calls PyObject_Del()

http://bugs.python.org/issue3299#msg78740
https://mail.python.org/pipermail/python-dev/2003-February/033258.html

_imagingft.c

@@ -153,7 +153,7 @@ getfont(PyObject* self_, PyObject* args, PyObject* kw)
         if (self->font_bytes) {
             PyMem_Free(self->font_bytes);
         }
-        PyObject_Del(self);
+        Py_DECREF(self);
         return geterror(error);
     }
 

map.c

@@ -73,7 +73,7 @@ PyImaging_MapperNew(const char* filename, int readonly)
         NULL);
     if (mapper->hFile == (HANDLE)-1) {
         PyErr_SetString(PyExc_IOError, "cannot open file");
-        PyObject_Del(mapper);
+        Py_DECREF(mapper);
         return NULL;
     }
 
@@ -84,7 +84,7 @@ PyImaging_MapperNew(const char* filename, int readonly)
     if (mapper->hMap == (HANDLE)-1) {
         CloseHandle(mapper->hFile);
         PyErr_SetString(PyExc_IOError, "cannot map file");
-        PyObject_Del(mapper);
+        Py_DECREF(mapper);
         return NULL;
     }