OP_ASGN vsp may be negative

reported by https://gist.github.com/miura1729/53fbd8af889c289a79108e38635b2378
fix proposed by @miura1729 in
https://github.com/miura1729/mruby/commit/b1b7933f7aa950cfb747b06327a0d0340f3e4ff8

mrbgems/mruby-compiler/core/codegen.c

@@ -1785,7 +1785,7 @@ codegen(codegen_scope *s, node *tree, int val)
         int pos;
 
         pop();
-        if (val) {
+        if (val && vsp >= 0) {
           genop(s, MKOP_AB(OP_MOVE, vsp, cursp()));
           pos = genop(s, MKOP_AsBx(name[0]=='|'?OP_JMPIF:OP_JMPNOT, cursp(), 0));
         }
@@ -1794,7 +1794,7 @@ codegen(codegen_scope *s, node *tree, int val)
         }
         codegen(s, tree->cdr->cdr->car, VAL);
         pop();
-        if (val) {
+        if (val && vsp >= 0) {
           genop(s, MKOP_AB(OP_MOVE, vsp, cursp()));
         }
         if ((intptr_t)tree->car->car == NODE_CALL) {
@@ -1855,7 +1855,7 @@ codegen(codegen_scope *s, node *tree, int val)
         gen_assignment(s, tree->car, cursp(), val);
       }
       else {
-        if (val) {
+        if (val && vsp >= 0) {
           genop(s, MKOP_AB(OP_MOVE, vsp, cursp()));
         }
         if (callargs == CALL_MAXARGS) {