Fix unsafe peephole optimization

Reported by https://hackerone.com/dkasak

Author: fbogsany

Diff for: mrbgems/mruby-compiler/core/codegen.c

@@ -1798,8 +1798,10 @@ codegen(codegen_scope *s, node *tree, int val)
         int pos;
 
         pop();
-        if (val && vsp >= 0) {
-          genop(s, MKOP_AB(OP_MOVE, vsp, cursp()));
+        if (val) {
+          if (vsp >= 0) {
+            genop(s, MKOP_AB(OP_MOVE, vsp, cursp()));
+          }
           pos = genop(s, MKOP_AsBx(name[0]=='|'?OP_JMPIF:OP_JMPNOT, cursp(), 0));
         }
         else {

Diff for: test/t/codegen.rb

@@ -0,0 +1,11 @@
+##
+# Codegen tests
+
+assert('peephole optimization does not eliminate move whose result is reused') do
+  assert_raise LocalJumpError do
+    def method
+      yield
+    end
+    method(&a &&= 0)
+  end
+end