Don't generate code for NODE_NEGATE if the result isn't used

Reported by https://hackerone.com/haquaman

mrbgems/mruby-compiler/core/codegen.c

@@ -2223,6 +2223,10 @@ codegen(codegen_scope *s, node *tree, int val)
     {
       nt = (intptr_t)tree->car;
       tree = tree->cdr;
+      if (!val) {
+        codegen(s, tree, NOVAL);
+        break;
+      }
       switch (nt) {
       case NODE_FLOAT:
         {

test/t/codegen.rb

@@ -63,3 +63,13 @@ assert('splat in case splat') do
 
   assert_equal [1], a
 end
+
+assert('negate literal register alignment') do
+  a = *case
+  when 0
+    -0.0
+    2
+  end
+
+  assert_equal [2], a
+end