aio_qphysio(): Avoid uninitialized pointer read on error.

For the !unmap case it may happen that pbuf gets called unreferenced
when vm_fault_quick_hold_pages() fails.
Initialize it so it doesn't cause trouble.

CID:		1352776
Reviewed by:	jhb
MFC after:	1 week

sys/kern/vfs_aio.c

@@ -1226,7 +1226,8 @@ aio_qphysio(struct proc *p, struct kaiocb *job)
 	if (!unmap) {
 		job->pbuf = pbuf = (struct buf *)getpbuf(NULL);
 		BUF_KERNPROC(pbuf);
-	}
+	} else
+		pbuf = NULL;
 
 	AIO_LOCK(ki);
 	if (!unmap)