Fix unsuccessfull rebalance operation

At the time irqd tries to rebalance singlequeue NICs to more CPUs,
but fails to do so (e. g. because already all CPUs being used)
there is a segfault:

  Program received signal SIGSEGV, Segmentation fault.
  0x0804b3c3 in queue_map_cpu (qi=0x886c1d8) at strategy.c:145

The fix is to jump out in this case, not doing any rebalancing.

Signed-off-by: Holger Eitzenberger <holger@eitzenberger.org>