Fix segfault when using result of rest assignment

Reported by https://hackerone.com/haquaman

mrbgems/mruby-compiler/core/codegen.c

@@ -1061,7 +1061,9 @@ gen_vmassignment(codegen_scope *s, node *tree, int rhs, int val)
         n++;
       }
     }
-    push();
+    if (!val) {
+      push();
+    }
   }
 }
 

test/t/codegen.rb

@@ -54,3 +54,12 @@ A
 B
   assert_equal "\n", a
 end
+
+assert('splat in case splat') do
+  a = *case
+    when 0
+      * = 1
+  end
+
+  assert_equal [1], a
+end