defrag: avoid passing null pointers to functions
DecodeThreadVars, ThreadVars, and possibly some other variables are sometimes passed as null parameters
in our unit test functions.
This leads to the necessity of checking if they're not null pointers in several other places, such as
seen in https://github.com/OISF/suricata/blob/master/src/defrag.c#L1033, before using those variables, to prevent
warnings code analyzers.
This may be the case for other files, too.
Expected result: clean up calls of functions with null pointers, so we can clean up those checks for NULL pointers.
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