Use assert instead of if without else to ensure an object pointer is not NULL.

Use assert instead of if without else to ensure an object pointer is not NULL.

gcc complained that a variable might be used uninitialized, which is
true. If NULL is passed, all code gets executed with an
uninitialized variable which is an error in almost every case.
Since python itself never passes NULL, only a real error in
other c code could trigger the assert.