BUG/MEDIUM: lua: segfault if a converter or a sample doesn't return anything

BUG/MEDIUM: lua: segfault if a converter or a sample doesn't return anything

In the case of a Lua sample-fetch or converter doesn't return any
value, an acces outside the Lua stack can be performed. This patch
check the stack size before converting the top value to a HAProxy
internal sample.

A workaround consist to check that a value value is always returned
with sample fetches and converters.

This patch should be backported in the version 1.6 and 1.7