libavb: Fix build warnings after updating the lib

libavb: Fix build warnings after updating the lib

After updating libavb to most recent version from AOSP/master, two new
warnings appear:

Warning #1:

    lib/libavb/avb_cmdline.c: In function 'avb_append_options':
    lib/libavb/avb_cmdline.c:365:15: warning: 'dm_verity_mode' may be
                                     used uninitialized in this function
                                     [-Wmaybe-uninitialized]
         new_ret = avb_replace(
                   ^~~~~~~~~~~~
             slot_data->cmdline, "$(ANDROID_VERITY_MODE)", dm_verity_mode);
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    lib/libavb/avb_cmdline.c:374:8: warning: 'verity_mode' may be used
                                    uninitialized in this function
                                    [-Wmaybe-uninitialized]
       if (!cmdline_append_option(
            ^~~~~~~~~~~~~~~~~~~~~~
               slot_data, "androidboot.veritymode", verity_mode)) {
               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Warning #2:

    lib/libavb/avb_slot_verify.c: In function 'avb_slot_verify':
    lib/libavb/avb_slot_verify.c:1349:23: warning: 'ret' may be used
                                          uninitialized in this function
                                          [-Wmaybe-uninitialized]
       AvbSlotVerifyResult ret;
                           ^~~

Fix those by providing default return values to affected functions.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>