parse.y: `cons_free` unused node (empty string node).

mrbgems/mruby-compiler/core/parse.y

@@ -3326,6 +3326,9 @@ string_fragment : tCHAR
                       if (intn($3->cdr->cdr) > 0) {
                         n = push(n, $3);
                       }
+                      else {
+                        cons_free($3);
+                      }
                       $$ = new_dstr(p, n);
                     }
                 ;
@@ -3372,6 +3375,9 @@ xstring         : tXSTRING_BEG tXSTRING
                       if (intn($3->cdr->cdr) > 0) {
                         n = push(n, $3);
                       }
+                      else {
+                        cons_free($3);
+                      }
                       $$ = new_dxstr(p, n);
                     }
                 ;
@@ -3439,6 +3445,9 @@ words           : tWORDS_BEG tSTRING
                       if (intn($3->cdr->cdr) > 0) {
                         n = push(n, $3);
                       }
+                      else {
+                        cons_free($3);
+                      }
                       $$ = new_words(p, n);
                     }
                 ;
@@ -3456,6 +3465,9 @@ symbol          : basic_symbol
                       if (intn($4->cdr->cdr) > 0) {
                         n = push(n, $4);
                       }
+                      else {
+                        cons_free($4);
+                      }
                       $$ = new_dsym(p, new_dstr(p, n));
                     }
                 ;