fix parse statement execute proto bug

[xuannnaux]

According to COM_STMT_EXECUTE protocol definition: https://dev.mysql.com/doc/internals/en/com-stmt-execute.html

      if new-params-bound-flag == 1:
    n              type of each parameter, length: num-params * 2
    n              value of each parameter

new-params-bound-flag condition only valid for the first statement type of each parameter.

value of each parameter field should be parsed every time.
Otherwise, the program will visit invalid memory address during the second STMT_EXECUTE.