Silence CID 329157: Dereference before null check in lib/dns/journal.c

*** CID 329157:  Null pointer dereferences  (REVERSE_INULL)
    /lib/dns/journal.c: 754 in journal_open()
    748     			    j->header.index_size * sizeof(journal_rawpos_t));
    749     	}
    750     	if (j->index != NULL) {
    751     		isc_mem_put(j->mctx, j->index,
    752     			    j->header.index_size * sizeof(journal_pos_t));
    753     	}
       CID 329157:  Null pointer dereferences  (REVERSE_INULL)
       Null-checking "j->filename" suggests that it may be null, but it has already been dereferenced on all paths leading to the check.
    754     	if (j->filename != NULL) {
    755     		isc_mem_free(j->mctx, j->filename);
    756     	}
    757     	if (j->fp != NULL) {
    758     		(void)isc_stdio_close(j->fp);
    759     	}

(cherry picked from commit 40544059)

Closes #2560 (closed)