[PARISC] fix panic on prefetch(NULL) on PA7300LC

Due to an errata, the PA7300LC generates a TLB miss interruption even on the
prefetch instruction.  This means that prefetch(NULL), which is supposed to be
a nop on linux actually generates a NULL deref fault.  Fix this by testing the
address of prefetch against NULL before doing the prefetch.

Cc: stable@vger.kernel.org
Signed-off-by: James Bottomley <JBottomley@Parallels.com>