Revert "Revert "[Fix] `set -u`: ensure that potentially unbound variables aren’t called unchecked.""

This reverts commit a8c465018e.

Fixes #1363.

nvm.sh

@@ -379,7 +379,7 @@ nvm_remote_versions() {
   NVM_LS_REMOTE_PRE_MERGED_OUTPUT=''
   local NVM_LS_REMOTE_POST_MERGED_OUTPUT
   NVM_LS_REMOTE_POST_MERGED_OUTPUT=''
-  if [ -z "${NVM_FLAVOR}" ] || [ "${NVM_FLAVOR}" = "${NVM_NODE_PREFIX}" ]; then
+  if [ -z "${NVM_FLAVOR-}" ] || [ "${NVM_FLAVOR-}" = "${NVM_NODE_PREFIX}" ]; then
     local NVM_LS_REMOTE_OUTPUT
     NVM_LS_REMOTE_OUTPUT=$(NVM_LTS="${NVM_LTS-}" nvm_ls_remote "${PATTERN-}") &&:
     NVM_LS_REMOTE_EXIT_CODE=$?
@@ -391,8 +391,8 @@ nvm_remote_versions() {
   local NVM_LS_REMOTE_IOJS_EXIT_CODE
   NVM_LS_REMOTE_IOJS_EXIT_CODE=0
   local NVM_LS_REMOTE_IOJS_OUTPUT
-  if [ -z "${NVM_LTS}" ] && ( \
-    [ -z "${NVM_FLAVOR}" ] || [ "${NVM_FLAVOR}" = "${NVM_IOJS_PREFIX}" ] \
+  if [ -z "${NVM_LTS-}" ] && ( \
+    [ -z "${NVM_FLAVOR-}" ] || [ "${NVM_FLAVOR-}" = "${NVM_IOJS_PREFIX}" ] \
   ); then
     NVM_LS_REMOTE_IOJS_OUTPUT=$(nvm_ls_remote_iojs "${PATTERN-}") &&:
     NVM_LS_REMOTE_IOJS_EXIT_CODE=$?
@@ -2406,6 +2406,7 @@ nvm() {
       fi
 
       local NVM_INSTALL_SUCCESS
+      NVM_INSTALL_SUCCESS=false
       if [ -n "${NVM_INSTALL_THIRD_PARTY_HOOK-}" ]; then
         nvm_err '** $NVM_INSTALL_THIRD_PARTY_HOOK env var set; dispatching to third-party installation method **'
         local NVM_METHOD_PREFERENCE