add KV Table iterator bool operator

libraries/eosiolib/contracts/eosio/key_value.hpp

@@ -444,6 +444,8 @@ namespace internal {
          return eosio::key_type{(char*)buffer, actual_value_size};
       }
 
+      bool valid() const { return itr_stat == status::iterator_ok; }
+
    protected:
       uint32_t itr;
       status itr_stat;
@@ -569,6 +571,10 @@ class table : internal::table_base {
       bool operator>=(const iterator& b) const {
          return base_iterator::compare(b) >= 0;
       }
+
+      explicit operator bool() const {
+         return this->valid();
+      }
    };
 
    class reverse_iterator : public base_iterator {