silence false positive errors from some compilers

Fixes #3529

src/builtin.cpp

@@ -2859,6 +2859,8 @@ static const wcstring hist_cmd_to_string(hist_cmd_t hist_cmd) {
         case HIST_SAVE:
             return L"save";
     }
+
+    DIE("should not reach this statement");  // silence some compiler errors about not returning
 }
 
 /// Remember the history subcommand and disallow selecting more than one history subcommand.

src/tokenizer.cpp

@@ -765,6 +765,8 @@ bool move_word_state_machine_t::consume_char(wchar_t c) {
             return consume_char_whitespace(c);
         }
     }
+
+    DIE("should not reach this statement");  // silence some compiler errors about not returning
 }
 
 move_word_state_machine_t::move_word_state_machine_t(move_word_style_t syl)