mruby-compiler: fix crash in pattern matching with string literal

The p_value grammar rule passed raw tSTRING token (a (len . str) cons
cell) directly to new_pat_value() without wrapping it as a proper AST
node. When codegen processed this malformed node, it read the length
field as the node type, causing misinterpretation and crash.

Wrap tSTRING with new_str(p, list1($1)) to create a proper NODE_STR,
consistent with how the primary:string rule handles strings.

Found by ClusterFuzz (oss-fuzz/mruby_fuzzer).

Co-authored-by: Claude <noreply@anthropic.com>

Author: matz

mrbgems/mruby-compiler/core/parse.y

@@ -3986,7 +3986,7 @@ p_value         : p_var
                     }
                 | tSTRING
                     {
-                      $$ = new_pat_value(p, $1);
+                      $$ = new_pat_value(p, new_str(p, list1($1)));
                     }
                 | keyword_nil
                     {

mrbgems/mruby-compiler/core/y.tab.c

@@ -10540,7 +10540,7 @@ YYLTYPE yylloc = yyloc_default;
   case 456: /* p_value: tSTRING  */
 #line 3988 "mrbgems/mruby-compiler/core/parse.y"
                     {
-                      (yyval.nd) = new_pat_value(p, (yyvsp[0].nd));
+                      (yyval.nd) = new_pat_value(p, new_str(p, list1((yyvsp[0].nd))));
                     }
 #line 10546 "mrbgems/mruby-compiler/core/y.tab.c"
     break;