If you’re going to check for NULL pointer arguments where you have not entered into a contract to accept and interpret them, do it with an assert, not a conditional error return. This way the bugs in the caller will be immediately detected and can be fixed, and it makes it easy to disable the overhead in production builds. I question the value of the assert except as documentation however; a segfault from dereferencing the NULL pointer is just as effective for debugging.
If you return an error code to a caller which has already proven itself buggy, the most likely result is that the caller will ignore the error, and bad things will happen much later down the line when the original cause of the error has become difficult or impossible to track down. Why is it reasonable to assume the caller will ignore the error you return? Because the caller already ignored the error return of malloc or fopen or some other library-specific allocation function which returned NULL to indicate an error!