Branch b1_6

b=15288
i=wangdi, shadow

fix stride_len assertion.

when current ra window is consumed, and 2 consequent read requires to enlarge
the read ahead window, and the enlargement action met window_start 0,
window_len 80, because the latest read consumes the whole window content,
stride_offset is 80, so the left of current ra window is 0, and the assertion
of stride_len should be ">= 0" rather than "> 0"