Modify handling of OP_ENTER.

If `argc` is `-1`, `block` parameter is sometimes overwritten.

src/vm.c

@@ -1121,6 +1121,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
       }
       else {
         if (argv0 != argv) {
+          regs[len+1] = *blk; /* move block */
           memmove(&regs[1], argv, sizeof(mrb_value)*(m1+o)); /* m1 + o */
         }
         if (r) {                  /* r */
@@ -1129,7 +1130,9 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
         if (m2) {
           memmove(&regs[m1+o+r+1], &argv[argc-m2], sizeof(mrb_value)*m2);
         }
-        regs[len+1] = *blk; /* move block */
+        if (argv0 == argv) {
+          regs[len+1] = *blk; /* move block */
+        }
         pc += o + 1;
       }
       JUMP;