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.