Initialize paddr to handle the case of zero size.

Reported and reviewed by:	Conrad Meyer <cemeyer@uw.edu>
MFC after:	1 week

sys/vm/device_pager.c

@@ -414,6 +414,7 @@ old_dev_pager_ctor(void *handle, vm_ooffset_t size, vm_prot_t prot,
 	 * XXX assumes VM_PROT_* == PROT_*
 	 */
 	npages = OFF_TO_IDX(size);
+	paddr = 0; /* Make paddr initialized for the case of size == 0. */
 	for (off = foff; npages--; off += PAGE_SIZE) {
 		if (csw->d_mmap(dev, off, &paddr, (int)prot, &dummy) != 0) {
 			dev_relthread(dev, ref);