hw/display/omap_lcdc: Fix potential NULL pointer dereference

In omap_lcd_interrupts(), the pointer omap_lcd is dereferinced before
being check if it is valid, which may lead to NULL pointer dereference.
So move the assignment to surface after checking that the omap_lcd is valid
and move surface_bits_per_pixel(surface) to after the surface assignment.

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: AlexChen <alex.chen@huawei.com>
Message-id: 5F9CDB8A.9000001@huawei.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/display/omap_lcdc.c

@@ -78,14 +78,18 @@ static void omap_lcd_interrupts(struct omap_lcd_panel_s *s)
 static void omap_update_display(void *opaque)
 {
     struct omap_lcd_panel_s *omap_lcd = (struct omap_lcd_panel_s *) opaque;
-    DisplaySurface *surface = qemu_console_surface(omap_lcd->con);
+    DisplaySurface *surface;
     draw_line_func draw_line;
     int size, height, first, last;
     int width, linesize, step, bpp, frame_offset;
     hwaddr frame_base;
 
-    if (!omap_lcd || omap_lcd->plm == 1 || !omap_lcd->enable ||
-        !surface_bits_per_pixel(surface)) {
+    if (!omap_lcd || omap_lcd->plm == 1 || !omap_lcd->enable) {
+        return;
+    }
+
+    surface = qemu_console_surface(omap_lcd->con);
+    if (!surface_bits_per_pixel(surface)) {
         return;
     }