Minor code clean-up for GCC 4.9.
Minor code clean-up for GCC 4.9.

- Tidy up "signed vs unsigned" comparison
- Unconditionally initialize the "protocol" variable. This is a
  false-positive warning as protocol is indeed initialized when used,
  but GCC failed to detect the dependency between protocol and ip6.

Change-Id: I8146898d91d45531c3de73403e0df3330c0a2043

jsnweitzel/android_external_android-clat,xin3liang/platform_external_android-clat,TeamExodus/external_android-clat,android-ia/platform_external_android-clat,android-ia/platform_external_android-clat,geekboxzone/mmallow_external_android-clat,thiz11/platform_external_android-clat,thiz11/platform_external_android-clat,geekboxzone/lollipop_external_android-clat,geekboxzone/lollipop_external_android-clat,Omegaphora/external_android-clat,jsnweitzel/android_external_android-clat,xin3liang/platform_external_android-clat,geekboxzone/mmallow_external_android-clat,Omegaphora/external_android-clat,TeamExodus/external_android-clat