Fix a race condition in QEvdevTabletHandler::readData()

Statics and threads don't mix well. There can be multiple threads
calling QEvdevTabletHandler::readData() simultaneously if you have
several tablet devices registered with the plugin, creating a race on
the static buffer array.

Make the buffer a simple local variable instead, the array is small
enough that we can afford the per-thread stack allocation.

Change-Id: I4487add8df50743b8178ca6faeb9be45231ccb78
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

src/platformsupport/input/evdevtablet/qevdevtablethandler.cpp

@@ -246,7 +246,7 @@ bool QEvdevTabletHandler::queryLimits()
 
 void QEvdevTabletHandler::readData()
 {
-    static input_event buffer[32];
+    input_event buffer[32];
     int n = 0;
     for (; ;) {
         int result = QT_READ(m_fd, reinterpret_cast<char*>(buffer) + n, sizeof(buffer) - n);