intermezzo: fix uninitialized use of pointer in error path

intermezzo: fix uninitialized use of pointer in error path

gcc pointed out the following issue :
  dcache.c: In function `presto_set_dd':
  dcache.c:251: warning: `fset' might be used uninitialized in this function

fset is not yet assigned in the error path, so no operation must be done
with it.