avb_str_concat: Fail if 0 is passed for buf_size. am: b73cea24f8 am: b5f275d3b5 am: 66992bbe14

Original change: https://googleplex-android-review.googlesource.com/c/platform/external/avb/+/10889377

Change-Id: I5f93e50e82e2e75509eca550697122ebad04574a