Don't access drive_g->desc from boot_cdrom().

The drive description is allocated with malloc_tmp() and is thus only
available during the POST phase - boot_cdrom() is called during the
boot phase.
diff --git a/src/boot.c b/src/boot.c
index a93bb5f..8e33d53 100644
--- a/src/boot.c
+++ b/src/boot.c
@@ -431,7 +431,7 @@
     struct drive_s *drive_g = (void*)ie->vector;
     int status = cdrom_boot(drive_g);
     if (status) {
-        printf("Boot failed: Could not read from CDROM %s (code %04x)\n", drive_g->desc, status);
+        printf("Boot failed: Could not read from CDROM (code %04x)\n", status);
         return;
     }