fix(measurex): make sure we don't redirect loop forever

This is the most immediate fix to the issue described by
ooni/probe#1792.

So, the logic was actually miss the increment, which
would have been noticed with proper unit testing.

Anyway, I am not sure why the loop ensues in the first
time. By looking at the headers, it seems we're passing
the headers correctly.

So, even though this fix interrupts the loop, it still
remains the question of whether the loop is legit or
whether we're missing extra logic to properly redirect.

internal/measurex/measurer.go

@@ -885,6 +885,7 @@ type redirectionQueue struct {
 
 func (r *redirectionQueue) append(URL ...string) {
 	r.q = append(r.q, URL...)
+	r.cnt++
 }
 
 func (r *redirectionQueue) popleft() (URL string) {