evaluate: Fix datalen checks in expr_evaluate_string()

evaluate: Fix datalen checks in expr_evaluate_string()

I have been told that the flex scanner won't return empty strings, so
strlen(data) should always be greater 0. To avoid a hard to debug issue
though, add an assert() to make sure this is always the case before
risking an unsigned variable underrun.

A real issue though is the check for 'datalen - 1 >= 0', which will
never fail due to datalen being unsigned. Fix this by incrementing both
sides by one, hence checking 'datalen >= 1'.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>