add missing fall through comments

mrbgems/mruby-compiler/core/parse.y

@@ -4802,6 +4802,7 @@ parser_yylex(parser_state *p)
   case ')':
   case ']':
     p->paren_nest--;
+    /* fall through */
   case '}':
     COND_LEXPOP();
     CMDARG_LEXPOP();
@@ -5133,6 +5134,7 @@ parser_yylex(parser_state *p)
         pushback(p,  c);
         return '$';
       }
+      /* fall through */
     case '0':
       tokadd(p, '$');
     }

mrbgems/mruby-string-utf8/src/string.c

@@ -259,6 +259,7 @@ mrb_str_aref(mrb_state *mrb, mrb_value str, mrb_value indx)
   switch (mrb_type(indx)) {
     case MRB_TT_FLOAT:
       indx = mrb_flo_to_fixnum(mrb, indx);
+      /* fall through */
     case MRB_TT_FIXNUM:
       idx = mrb_fixnum(indx);