AOF rewrite: set iterator var to NULL when freed.

The cleanup code expects that if 'di' is not NULL, it is a valid
iterator that should be freed.

The result of this bug was a crash of the AOF rewriting process if an
error occurred after the DBs data are written and the iterator is no
longer valid.

src/aof.c

@@ -1082,6 +1082,7 @@ int rewriteAppendOnlyFile(char *filename) {
             }
         }
         dictReleaseIterator(di);
+        di = NULL;
     }
 
     /* Do an initial slow fsync here while the parent is still sending