getline < directory returns zero

Both the POSIX spec and the OTA man page state that getline returns -1 for an error, but in practice OTA getline returns 0 when attempting to read from a directory. Despite the misleading 0 return value, it prints an error to stderr. ``` $ ./awk --version awk version 20240728 $ ./awk 'BEGIN { x = (getline < "/"); print x }' # would expect this to print -1 0 ./awk: i/o error occurred on / source line number 1 ```