BUG/MEDIUM: lua/socket: Length required read doesn't work

BUG/MEDIUM: lua/socket: Length required read doesn't work

The limit of data read works only if all the data is in the
input buffer. Otherwise (if the data arrive in chunks), the
total amount of data is not taken in acount.

Only the current read data are compared to the expected amout
of data.

This patch must be backported from 1.9 to 1.6