[Fix] check invalid NVM_DIR in install script

[AlightSoulmate]

Refs #1521

The install script now checks invalid $NVM_DIR paths and reports the first
invalid or missing directory.

Tests were added for this case.

[ljharb]

a few comments

[ljharb]

Two bugs here, both from reducing the path with ${INSTALL_DIR%/*}:

1. Infinite loop on a relative $NVM_DIR. On a slash-less component, ${foo%/*} returns foo unchanged, so this loop never makes progress and install.sh hangs (100% CPU, no output). The INSTALL_DIR="." fallback only fires for absolute single-component paths (where the strip yields ""), so it never rescues the relative case. Reachable in practice: NVM_DIR=mynvm bash install.sh → nvm_do_install → nvm_validate_install_dir → here. The previous code exited cleanly with an error for this input, so this is a regression to a worse failure mode.

2. Wrong base for an absolute path whose first component is missing. ${/a%/*} → "" → the . fallback, which is right for relative paths but wrong for absolute ones: nvm_install_dir_base /nonexistent/foo returns . instead of /nonexistent (or /), which surfaces as a ./… path in the error message below.

Suggested fix with dirname (reduces correctly — /a→/, foo→., a/b→a) plus a fixed-point break:

nvm_install_dir_base() {
  local INSTALL_DIR PARENT
  INSTALL_DIR="${1}"
  while [ ! -e "${INSTALL_DIR}" ]; do
    PARENT="$(command dirname "${INSTALL_DIR}")"
    [ "${PARENT}" = "${INSTALL_DIR}" ] && break
    INSTALL_DIR="${PARENT}"
  done
  printf %s "${INSTALL_DIR}"
}

[ljharb]

Symptom of the nvm_install_dir_base bug above: for an absolute $NVM_DIR whose first component doesn't exist, this emits a relative path. e.g. NVM_DIR=/nonexistentROOT123/foo prints failed to find directory "./nonexistentROOT123", but the missing directory is /nonexistentROOT123. Fixing the base helper resolves this.

[ljharb]

These cases only exercise bases that already exist - /dev/null (a file) and $(pwd) - so neither nvm_install_dir_base bug is caught, and the suite stays green while the helper is broken. Two cases worth adding:

• a relative, non-existent NVM_DIR (e.g. NVM_DIR=relnvm) - catches the infinite loop; run it under a timeout/watchdog so a hang fails the test instead of stalling CI.
• an absolute path whose first component is missing (e.g. NVM_DIR=/nonexistentROOT123/foo) - catches the ./… message bug.