[PATCH] hw: ide: check the pointer before do dma memory unmap

[Li Qiang <liq3ea@163.com>]

In 'map_page' we need to check the return value of
'dma_memory_map' to ensure the we actully maped something.
Otherwise, we will hit an assert in 'address_space_unmap'.
This is because we can't find the MR with the NULL buffer.
This is the LP#1884693:

-->https://bugs.launchpad.net/qemu/+bug/1884693

Reported-by: Alexander Bulekov <alxndr@bu.edu>
Signed-off-by: Li Qiang <liq3ea@163.com>
---
 hw/ide/ahci.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/hw/ide/ahci.c b/hw/ide/ahci.c
index 009120f88b..63e9fccdbe 100644
--- a/hw/ide/ahci.c
+++ b/hw/ide/ahci.c
@@ -250,6 +250,11 @@ static void map_page(AddressSpace *as, uint8_t **ptr, uint64_t addr,
     }
 
     *ptr = dma_memory_map(as, addr, &len, DMA_DIRECTION_FROM_DEVICE);
+
+    if (!*ptr) {
+        return;
+    }
+
     if (len < wanted) {
         dma_memory_unmap(as, *ptr, len, DMA_DIRECTION_FROM_DEVICE, len);
         *ptr = NULL;
-- 
2.17.1