Guard avifDiagnosticsClearError() in avif.c

[y-guyon]

[skip ci]

[wantehchang]

I have stated my preference. This pull request is one of two ways to make the two functions consistent about whether a null diag is allowed.

@joedrago The "calls to avifFileTypeIsCompatible()" part of the comment is incorrect. Joe, do you remember what you had in mind when you wrote it?

[y-guyon]

@joedrago The "calls to avifFileTypeIsCompatible()" part of the comment is incorrect. Joe, do you remember what you had in mind when you wrote it?

It was about avifPeekCompatibleFileType() and its BEGIN_STREAM() called with NULL diag argument:

libavif/src/read.c

Lines 3196 to 3198 in 3e66456

	avifBool avifPeekCompatibleFileType(const avifROData * input)
	{
	BEGIN_STREAM(s, input->data, input->size, NULL, NULL);

[joedrago]

My intent with avifFileTypeIsCompatible() was to be a very cheap way to peek into a buffer and see if it was likely to be an AVIF. If it internally creates an avifDiagnostics and throws away its value so that the entire codebase can assert that a diag pointer is never NULL, I think that is a good change.