Update inject_virtual_thread.cc (#806)

This compilation warning is fixed.
src/pass/inject_virtual_thread.cc:43:19: warning: ‘rw_mask’ may be used uninitialized in this function [-Wmaybe-uninitialized]
       if (rw_mask & 2) {
           ~~~~~~~~^~~

src/pass/inject_virtual_thread.cc

@@ -32,7 +32,7 @@ class ExprTouched final : public IRVisitor {
   }
   void Visit_(const Call *op) final {
     if (op->is_intrinsic(intrinsic::tvm_access_ptr)) {
-      int rw_mask;
+      int rw_mask = 0;
       CHECK(arith::GetConstInt(op->args[4], &rw_mask));
       const Variable* buffer_var = op->args[1].as<Variable>();
       CHECK(buffer_var);