[bug] handling case where prefix is NULL or 0 sized in refPrefix_advanced

[bimbashrestha]

Adding refPrefix to our dictionary_round_trip fuzzer caused it to crash on

zstd/lib/compress/zstd_compress.c

Lines 3003 to 3007 in dde98d8

	DEBUGLOG(4, "ZSTD_compress_insertDictionary (dictSize=%u)", (U32)dictSize);
	if ((dict==NULL) || (dictSize<8)) {
	RETURN_ERROR_IF(dictContentType == ZSTD_dct_fullDict, dictionary_wrong);
	return 0;
	}

handling null case on refPrefix advanced like in loadDictionary. I'll rebase the patch-from pr on top of this after it lands.

[Cyan4973]

nice catch !