[libinput] A not valid LibInput::Context should return -1 as filedescriptor

0 is a valid filedescriptor after all.

Autotest extended to cover this case.

autotests/libinput/context_test.cpp

@@ -54,6 +54,8 @@ void TestContext::testCreateFailUdev()
     // should not have a valid libinput
     libinput *libinput = context;
     QVERIFY(!libinput);
+    QVERIFY(!context.assignSeat("testSeat"));
+    QCOMPARE(context.fileDescriptor(), -1);
 }
 
 void TestContext::testAssignSeat_data()

libinput/context.cpp

@@ -81,7 +81,7 @@ bool Context::assignSeat(const char *seat)
 int Context::fileDescriptor()
 {
     if (!isValid()) {
-        return 0;
+        return -1;
     }
     return libinput_get_fd(m_libinput);
 }