Null checks before dereferencing a pointer

[elichai]

Hi,
As far as I could see this doesn't currently affect anything in the code but there's a pointer dereference here:
https://github.com/bitcoin-core/secp256k1/blob/master/src/ecmult_impl.h#L398

and only a few lines afterwards we check if it's not null
https://github.com/bitcoin-core/secp256k1/blob/master/src/ecmult_impl.h#L406

[elichai]

It seems that VERIFY_CHECK is for debug only,and if so this doesn't matter too much .

(haven't yet grasped all of the different checks here and what each one does under different flags)

[real-or-random]

Good catch. If we check it, then we should check it before we dereference it... (Otherwise the compiler may just remove the check.)

Do you want to open a PR?

Different checks:

• CHECK is always enabled, and calls the error callback on failure.
• VERIFY_CHECK is the same but only enabled in debug
• ARG_CHECK is always enabled, and calls the illegal (argument) callback.
• ARG_CHECK_NO_RETURN is like ARG_CHECK but for use in void functions

For the usage see #566 (comment), it's not 100% consistent throughout the library, and there are cases where it's not 100% clear what the right check is. As I interpret this guideline, it will acceptable to replace the VERIFY_CHECK by a CHECK here. But for the sake of doing only a minimal change, I recommend leaving it as it is and not caring too much about it.

[elichai]

Sure, opened a PR.