[PATCH synaptics] Avoid out-of-bounds access by running num_active_touches < 0 (#49439)

[Peter Hutterer]

If a touch is active during driver init, the slot will be set to
SLOTSTATE_CLOSE when it finishes. That could decrease num_active_touches to
less than 0, causing out-of-bounds access.

X.Org Bug 49439 <http://bugs.freedesktop.org/show_bug.cgi?id=49439>

Signed-off-by: Peter Hutterer <peter.hutterer at who-t.net>
---
 src/synaptics.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/synaptics.c b/src/synaptics.c
index e792977..d98ba71 100644
--- a/src/synaptics.c
+++ b/src/synaptics.c
@@ -2624,7 +2624,8 @@ UpdateTouchState(InputInfoPtr pInfo, struct SynapticsHwState *hw)
                     priv->open_slots[j] = priv->open_slots[j + 1];
             }
 
-            priv->num_active_touches--;
+            if (priv->num_active_touches > 0)
+                priv->num_active_touches--;
         }
     }
 
-- 
1.7.10.1