Do not call lfs_free if the buffer to free is NULL

[cdesjardins]

This can happen if lfs_malloc fails, or if the file doesn't exist, all "goto cleanup" clauses before and including the the lfs_malloc will cause this.

[vonj]

This can happen if lfs_malloc fails, or if the file doesn't exist, all "goto cleanup" clauses before and including the the lfs_malloc will cause this.

Can this be triggered in a unit test, or would a malloc fail somehow have to be provoked?

[BenBE]

Please note my comment in this discussion: free with NULL is a no-op. Only thing that littlefs should guarantee is to properly initialize all its memory so unset pointers are NULL instead of random values..

[evpopov]

I suggest this pull request be closed in lieu of #790 that I just created.
@cdesjardins, nothing personal, I've just been procrastinating after my discussion with @BenBE and just now managed to create a PR that addresses the same issue. I also covered the lfs_deinit() function in my PR which I believe covers all uses the lfs_free().

[geky]

It's worth noting @cdesjardins's case is a bit different from #790, since the case of the "goto cleanup" in lfs_file_open can be reached in normal operation.

That being said, as @BenBE points out, free with NULL is usually a no-op, and is guaranteed to be so in a standard-conforming stdlib.

And, if free does not support NULL, the lfs_free function is under user control if they create their own lfs_util.h file. I believe this would be the correct place to add such a NULL check.