[MIPS] mem->size from ioctl could be zero, protect against that.

New codec code seem to send a mem->size == 0 in an ioctl.
Kernel needs to protect itself. Can't call dma_cache_wback()
with a size of zero; it will panic with a stack that isn't
in good shape to see that the origin of the problem was here.

Signed-off-by: Pete Delaney <piet.delaney@imgtec.com>