Add NULL check for gen->gi_code in gen_send_ex()

_PyGen_Finalize() checks that gen->gi_code is not NULL before it
accesses the flags of the code object. This means that the flag
could be NULL.

It passes down the generatore to gen_close() and gen_send_ex().
gen_send_ex() did not check for gen->gi_code != NULL.

CID 1297900