fs_mgr: avb check avb_slot_data

Damaged avb metadata can result in avb_slot_verify returning a
nullptr in avb_slot_data.  Instead of an illegal access
violation in first_stage_init, we return the verify_result so
that it can be acted upon.

Test: confirm happenstance damaged vbmeta does not crash init.
Change-Id: I15be5bd32760bcc3418c5d8a943b016c0ddd56bc
diff --git a/fs_mgr/libfs_avb/avb_ops.cpp b/fs_mgr/libfs_avb/avb_ops.cpp
index 3b0ef0b..3efa794 100644
--- a/fs_mgr/libfs_avb/avb_ops.cpp
+++ b/fs_mgr/libfs_avb/avb_ops.cpp
@@ -186,6 +186,7 @@
             avb_slot_verify(&avb_ops_, requested_partitions, ab_suffix.c_str(), flags,
                             AVB_HASHTREE_ERROR_MODE_RESTART_AND_INVALIDATE, &avb_slot_data);
 
+    if (!avb_slot_data) return verify_result;
     // Copies avb_slot_data->vbmeta_images[].
     for (size_t i = 0; i < avb_slot_data->num_vbmeta_images; i++) {
         out_vbmeta_images->emplace_back(VBMetaData(avb_slot_data->vbmeta_images[i].vbmeta_data,