bochs-display: Fix vgamem=SIZE error handling

bochs_display_realize() rejects out-of-range vgamem.  The error
handling is broken:

    $ qemu-system-x86_64 -S -display none -monitor stdio
    QEMU 4.2.93 monitor - type 'help' for more information
    (qemu) device_add bochs-display,vgamem=1
    Error: bochs-display: video memory too small
    (qemu) device_add bochs-display,vgamem=1
    RAMBlock "0000:00:04.0/bochs-display-vram" already registered, abort!
    Aborted (core dumped)

Cause: bochs_display_realize() neglects to bail out after setting the
error.  Fix that.

Fixes: 765c942
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200422130719.28225-8-armbru@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>

Author: Markus Armbruster

hw/display/bochs-display.c

@@ -267,16 +267,18 @@ static void bochs_display_realize(PCIDevice *dev, Error **errp)
     Object *obj = OBJECT(dev);
     int ret;
 
-    s->con = graphic_console_init(DEVICE(dev), 0, &bochs_display_gfx_ops, s);
-
     if (s->vgamem < 4 * MiB) {
         error_setg(errp, "bochs-display: video memory too small");
+        return;
     }
     if (s->vgamem > 256 * MiB) {
         error_setg(errp, "bochs-display: video memory too big");
+        return;
     }
     s->vgamem = pow2ceil(s->vgamem);
 
+    s->con = graphic_console_init(DEVICE(dev), 0, &bochs_display_gfx_ops, s);
+
     memory_region_init_ram(&s->vram, obj, "bochs-display-vram", s->vgamem,
                            &error_fatal);
     memory_region_init_io(&s->vbe, obj, &bochs_display_vbe_ops, s,