fix bug when pulling small msg from a larger number of servers in dyn_val
fix bug when pulling small msg from a larger number of servers in
dyn_val

shilad/ps-lite,dmlc/ps-lite,dmlc/ps-lite,xcgoner/ps-lite-new,yuanxiaolong/ps-lite,xcgoner/ps-lite-new,toothacher17/ps-lite,shilad/ps-lite,xcgoner/ps-lite-new,xindaya/ps-lite,dmlc/ps-lite,toothacher17/ps-lite,xindaya/ps-lite,shilad/ps-lite,yuanxiaolong/ps-lite,xcgoner/ps-lite-new,dmlc/ps-lite