pc_sysfw: Plug memory leak on pc_fw_add_pflash_drv() error path

Harmless, because we the error inevitably leads to another, fatal one
in pc_system_flash_init(): PC system firmware (pflash) not available.
Fix it anyway.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

hw/pc_sysfw.c

@@ -98,7 +98,9 @@ static void pc_fw_add_pflash_drv(void)
       return;
     }
 
-    drive_init(opts, machine->use_scsi);
+    if (!drive_init(opts, machine->use_scsi)) {
+        qemu_opts_del(opts);
+    }
 }
 
 static void pc_system_flash_init(MemoryRegion *rom_memory,