Fix assert Crash when code size is 0

I encountered a crash from the asertion line (e.g. 1240) when running a Cython generated extension module (grpc-python, which creates async def functions) under PYTHONDEBUGMODE.

It appears that the co object created by Cython in this case has a zero length code bytes -- which probably made sense because there is no python bytecode.

Assuming Cython is correct in reporting a non-zero sized co object in this case, I think we can short circuit the Addr* functions for zero sized co objects.

Author: rainwoodman

Objects/codeobject.c

@@ -1015,7 +1015,7 @@ PyCode_NewEmpty(const char *filename, const char *funcname, int firstlineno)
 int
 PyCode_Addr2Line(PyCodeObject *co, int addrq)
 {
-    if (addrq < 0) {
+    if (addrq < 0 || _PyCode_NBYTES(co)) {
         return co->co_firstlineno;
     }
     if (co->_co_monitoring && co->_co_monitoring->lines) {
@@ -1232,7 +1232,7 @@ PyCode_Addr2Location(PyCodeObject *co, int addrq,
                      int *start_line, int *start_column,
                      int *end_line, int *end_column)
 {
-    if (addrq < 0) {
+    if (addrq < 0 || _PyCode_NBYTES(co) == 0) {
         *start_line = *end_line = co->co_firstlineno;
         *start_column = *end_column = 0;
         return 1;