Use correct buffer size in string subscript operator.

src/wren_core.c

@@ -968,7 +968,7 @@ DEF_NATIVE(string_subscript)
 
   // The result is a one-character string.
   // TODO: Handle UTF-8.
-  Value value = wrenNewUninitializedString(vm, 2);
+  Value value = wrenNewUninitializedString(vm, 1);
   ObjString* result = AS_STRING(value);
   result->value[0] = AS_CSTRING(args[0])[index];
   result->value[1] = '\0';

src/wren_value.c

@@ -341,7 +341,7 @@ Value wrenNewUninitializedString(WrenVM* vm, size_t length)
 {
   ObjString* string = allocate(vm, sizeof(ObjString) + length + 1);
   initObj(vm, &string->obj, OBJ_STRING, vm->stringClass);
-  string->length = length;
+  string->length = (int)length;
 
   return OBJ_VAL(string);
 }