`nvm_strip_path`: Error out when `$NVM_DIR` is not set.

Per https://github.com/creationix/nvm/issues/1083#issuecomment-217752185

nvm.sh

@@ -403,6 +403,10 @@ nvm_num_version_groups() {
 }
 
 nvm_strip_path() {
+  if [ -z "${NVM_DIR-}" ]; then
+    nvm_err '$NVM_DIR not set!'
+    return 1
+  fi
   nvm_echo "$1" | command sed \
     -e "s#$NVM_DIR/[^/]*$2[^:]*:##g" \
     -e "s#:$NVM_DIR/[^/]*$2[^:]*##g" \