Avoid needless object allocation

struct.c

@@ -957,12 +957,15 @@ rb_struct_deconstruct_keys(VALUE s, VALUE keys)
                  rb_obj_class(keys));
 
     }
+    if (RSTRUCT_LEN(s) < RARRAY_LEN(keys)) {
+        return rb_hash_new_with_size(0);
+    }
     h = rb_hash_new_with_size(RARRAY_LEN(keys));
     for (i=0; i<RARRAY_LEN(keys); i++) {
         VALUE key = RARRAY_AREF(keys, i);
         int i = rb_struct_pos(s, &key);
         if (i < 0) {
-            return rb_hash_new_with_size(0);
+            return h;
         }
         rb_hash_aset(h, key, RSTRUCT_GET(s, i));
     }

test/ruby/test_pattern_matching.rb

@@ -1250,6 +1250,8 @@ END
       case s[a: 0, b: 1]
       in a:, c:
         flunk
+      in a:, b:, c:
+        flunk
       in b:
         b == 1
       end

test/ruby/test_struct.rb

@@ -437,7 +437,7 @@ module TestStruct
     assert_equal({a: 1, b: 2}, o.deconstruct_keys(nil))
     assert_equal({a: 1, b: 2}, o.deconstruct_keys([:b, :a]))
     assert_equal({a: 1}, o.deconstruct_keys([:a]))
-    assert_equal({}, o.deconstruct_keys([:a, :c]))
+    assert_not_send([o.deconstruct_keys([:a, :c]), :key?, :c])
     assert_raise(TypeError) {
       o.deconstruct_keys(0)
     }