Test for case when challenge_altnames is empty (#321)

When all names in a cert have already been validated, the challenge_altnames array will be empty, causes an error in later code. This patch adds a test to handle that case.

dehydrated

@@ -520,6 +520,7 @@ sign_csr() {
   # Respond to challenges
   reqstatus="valid"
   idx=0
+  if [ ${#challenge_altnames[@]} -ne 0 ]; then
     for altname in "${challenge_altnames[@]:0}"; do
       challenge_token="${challenge_tokens[${idx}]}"
       keyauth="${keyauths[${idx}]}"
@@ -555,6 +556,7 @@ sign_csr() {
         [[ -n "${HOOK}" ]] && [[ "${HOOK_CHAIN}" != "yes" ]] && "${HOOK}" "invalid_challenge" "${altname}" "${result}"
       fi
     done
+  fi
 
   # Wait for hook script to clean the challenges if used
   # shellcheck disable=SC2068