add guard to may_select in case stdin is not a terminal

pbc

@@ -180,8 +180,12 @@ function may_select() {
   if [ -z "$value" ]; then
     if (( $# == 1 )); then
       value=$1
+    elif [ ! -t 0 ]; then
+      echo "Error: no value given and stdin is not a terminal" >&2
+      return 1
     else
-      local sorted=($(printf "%s\n" "$@" | sort))
+      local sorted
+      readarray -t sorted < <(printf '%s\n' "$@" | sort)
       select value in "${sorted[@]}"; do
         [ -n "$value" ] && break
       done
@@ -190,8 +194,8 @@ function may_select() {
     return 1
   fi
 
+  [ -z "$value" ] && return 1
   echo "$value"
-  return 0
 }