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.

src/stick_table.c

@@ -496,15 +496,8 @@ struct stktable_key *smp_to_stkey(struct sample *smp, struct stktable *t)
 		break;
 
 	case SMP_T_STR:
-		/* Must be NULL terminated. */
-		if (smp->data.u.str.len >= smp->data.u.str.size ||
-		    smp->data.u.str.str[smp->data.u.str.len] != '\0') {
-			if (!smp_dup(smp))
-				return NULL;
-			if (smp->data.u.str.len >= smp->data.u.str.size)
-				return NULL;
-			smp->data.u.str.str[smp->data.u.str.len] = '\0';
-		}
+		if (!smp_make_safe(smp))
+			return NULL;
 		static_table_key->key = smp->data.u.str.str;
 		static_table_key->key_len = smp->data.u.str.len;
 		break;