Fix the potential segmentation fault

Extral newline can trigger recovery segmentation fault
Test case:
host$ adb shell 'echo -en "--update_package=ota_update.zip\n--show_text\n\n" > /cache/recovery/command'
host$ adb reboot recovery

Change-Id: If1781c1f5ad94a273f1cb122b67cedd9fb562433
Signed-off-by: Jin Feng <jin88.feng@gmail.com>