vxp: fix vslq_test_rec() for FreeBSD

the actual issue was that strtoll(".456", ...) sets errno 22, so when
we fall back to strtod(), we need to clear it.

Ref 0ca8d17
Ref 0dfa3b8

Fixes #4088

Yes, I did install FreeBSD, and it's been a nice refreshing no-frills experience
so far.

lib/libvarnishapi/vsl_query.c

@@ -211,6 +211,7 @@ vslq_test_rec(const struct vex *vex, const struct VSLC_ptr *rec)
 			lhs_int = strtoll(b, &q, 0);
 			AN(q);
 			if (q != e && (*q == '.' || *q == 'e')) {
+				errno = 0;
 				lhs_float = strtod(b, &q);
 				lhs_int = (long long)lhs_float;
 				lhs_float = 0.;