powerpc/pmap: Account for a potential NULL pmap in pmap_sync_icache

It's apparently possible for pcpu->pc_curpmap to be NULL at some point,
leading to a panic. Account for this as is done with the other 64-bit
AIM pmap.

Reported by: pkubaj
Tested by: pkubaj
Fixes: 6f0b2a235a1 ("Add pmap_sync_icache() for radix pmap")
MFC after: 3 days