intel_iommu: Fix mask may be uninitialized in vtd_context_device_invalidate

With -Werror=maybe-uninitialized configuration we get
../hw/i386/intel_iommu.c: In function ‘vtd_context_device_invalidate’:
../hw/i386/intel_iommu.c:1888:10: error: ‘mask’ may be used
uninitialized in this function [-Werror=maybe-uninitialized]
 1888 |     mask = ~mask;
      |     ~~~~~^~~~~~~

Add a g_assert_not_reached() to avoid the error.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20210309102742.30442-2-eric.auger@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/i386/intel_iommu.c

@@ -1884,6 +1884,8 @@ static void vtd_context_device_invalidate(IntelIOMMUState *s,
     case 3:
         mask = 7;   /* Mask bit 2:0 in the SID field */
         break;
+    default:
+        g_assert_not_reached();
     }
     mask = ~mask;