Avoid nullptr math undefined behaviour in __Pyx_PyList_GetSlice() utility function (GH-4734)

Some code I have uses lxml. When I compile and run lxml with LLVM-12's undefined behaviour sanitizer, I see this:
```
lxml/4.6.3/cython_parts=etree.c/etree.c:191228:65: runtime error: applying zero offset to null pointer
    #0 0x7faedc799e7c in __Pyx_PyList_GetSlice lxml/4.6.3/cython_parts=etree.c/etree.c:191228
    #1 0x7faedc6b3723 in __pyx_f_4lxml_5etree_9_ErrorLog_copy lxml/4.6.3/cython_parts=etree.c/etree.c:38092
    #2 0x7faedc77636e in __pyx_f_4lxml_5etree___copyGlobalErrorLog lxml/4.6.3/cython_parts=etree.c/etree.c:40203
    #3 0x7faedc775a5c in __pyx_pf_4lxml_5etree_9LxmlError___init__ lxml/4.6.3/cython_parts=etree.c/etree.c:21080
    #4 0x7faedc774dff in __pyx_pw_4lxml_5etree_9LxmlError_1__init__ lxml/4.6.3/cython_parts=etree.c/etree.c:21019
```
which implies that Cython's generating undefined behaviour.

Making an early exit from the modified function (essentially by hoisting an early exit from one level lower in the stack) avoids this.

Cython/Utility/ObjectHandling.c

@@ -857,6 +857,12 @@ static CYTHON_INLINE PyObject* __Pyx_Py{{type}}_GetSlice(
             PyObject* src, Py_ssize_t start, Py_ssize_t stop) {
     Py_ssize_t length = Py{{type}}_GET_SIZE(src);
     __Pyx_crop_slice(&start, &stop, &length);
+{{if type=='List'}}
+    if (length == 0) {
+        // Avoid undefined behaviour when accessing `ob_item` of an empty list.
+        return PyList_New(0);    
+    }
+{{endif}}
     return __Pyx_Py{{type}}_FromArray(((Py{{type}}Object*)src)->ob_item + start, length);
 }
 {{endfor}}