exec: Stop using memory after free

memory_region_unref(mr) can free memory.

For example I got:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7f43280d4700 (LWP 4462)]
0x00007f43323283c0 in phys_section_destroy (mr=0x7f43259468b0)
    at /home/don/xen/tools/qemu-xen-dir/exec.c:1023
1023        if (mr->subpage) {
(gdb) bt
    at /home/don/xen/tools/qemu-xen-dir/exec.c:1023
    at /home/don/xen/tools/qemu-xen-dir/exec.c:1034
    at /home/don/xen/tools/qemu-xen-dir/exec.c:2205
(gdb) p mr
$1 = (MemoryRegion *) 0x7f43259468b0

And this change prevents this.
Signed-off-by: NDon Slutz <Don.Slutz@Gmail.com>
Message-Id: <1448921464-21845-1-git-send-email-Don.Slutz@Gmail.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

exec.c

@@ -1064,9 +1064,11 @@ static uint16_t phys_section_add(PhysPageMap *map,
 
 static void phys_section_destroy(MemoryRegion *mr)
 {
+    bool have_sub_page = mr->subpage;
+
     memory_region_unref(mr);
 
-    if (mr->subpage) {
+    if (have_sub_page) {
         subpage_t *subpage = container_of(mr, subpage_t, iomem);
         object_unref(OBJECT(&subpage->iomem));
         g_free(subpage);