fix(_comp_compgen_help): work around nounset

Author: akinomyoga

bash_completion

@@ -1243,7 +1243,7 @@ _comp_compgen_help()
                 _comp_dequote "$cmd" && LC_ALL=C "$ret" "${opt[@]}" 2>&1
                 ;;
         esac
-    )"
+    )" || return 1
 
     local -a _options=()
     local _line
@@ -1287,7 +1287,7 @@ _comp_compgen_usage()
                 _comp_dequote "$cmd" && LC_ALL=C "$ret" "${opt[@]}" 2>&1
                 ;;
         esac
-    )"
+    )" || return 1
 
     local -a _options=()
     local _line _match _option _i _char