BUG/MEDIUM: stick-tables: do not fail on string keys with no allocated size

BUG/MEDIUM: stick-tables: do not fail on string keys with no allocated size

When a stick-table key is derived from a string-based sample, it checks
if it's properly zero-terminated otherwise tries to do so. But the test
doesn't work for two reasons :
  - the reported allocated size may be zero while the sample is maked as
    not CONST (eg: certain sample fetch functions like smp_fetch_base()
    do this), so smp_dup() prior to the recent changes will fail on this.

  - the string might have been converted from a binary sample, where the
    trailing zero is not appended. If the sample was writable, smp_dup()
    would not modify it either and we would fail again here. This may
    happen with req.payload or req.body_param for example.

The correct solution consists in calling smp_make_safe() to ensure the
sample is usable as a valid string.

This fix must be backported to 1.6.