AP_Filesystem: Change the judgment null of methods that return nullptr

[muramura]

I found myself using NULL in the decision of a method that returns NULLPTR.
I believe there is a difference between NULL and NULLPTR.
I will change the decision value from NULL to NULLPTR.

[hendjoshsr71]

Looks fine.

But you missed a few in the file. You might as well do them all at once. (I got 17 when I looked, Lines 67-80, Line 107.)

If you wanted to, there are a few style fixups you could do on a few lines since you changed the lines anyways.

[muramura]

@hendjoshsr71 san
Thanks for the comment.
I agreed.
I've adapted the style.

[hendjoshsr71]

Sorry if I was confusing. I meant there are still others spots with == NULL you could replace in this file. Using VSCode and this search == NULL I find the following still left.
Line 67
Line 69
Line 74
Line 80
Line107
Line 161

[muramura]

@hendjoshsr71 sam
The lines you mentioned are as follows.
The element value of the table and the return value of the method are null.
In particular, the C language methods are NULL.
The ArduPilot uses the return value of the method to make a decision.

Line 67: file_table[] returns NULL.
Line 69: The calloc method returns NULL.
Line 74: The calloc method returns NULL.
Line 80: The malloc method returns NULL.
Line 107: file_table[] returns NULL.
Line 161: file_table[].fh returns NULL.

[hendjoshsr71]

I'm not a C++ expert, but I did come across this https://stackoverflow.com/questions/53532697/c-style-when-using-c-function-in-c-should-i-check-for-null-or-nullptr

Either seems fine and what you have here is better than before :)

But lets ask @peterbarker since I've heard he is a resident C++ expert recently ;)

[khancyr]

yep, that would be great to get rid of the NULL from the codebase.
On nice things, beside off some cpp compile time check is that it also remove a lot of noise on static analysers!