[v15] Fix panic in app dialing (#42786)

* Fix panic in app dialing

#35501 incorrectly
checked the length on the local servers variable instead of
`t.c.servers` which could lead to panics like the one below.

```bash

panic: runtime error: slice bounds out of range [1:0]
goroutine 6558252 [running]:
github.com/gravitational/teleport/lib/web/app.(*transport).DialContext(0xc00296d5f0, {0xae8bbd8, 0xc002596a20}, {0x42c525?, 0xc001dc4d80?}, {0x120?, 0x118?})
    github.com/gravitational/teleport/lib/web/app/transport.go:264 +0x5dc
net/http.(*Transport).dial(0xc002596a20?, {0xae8bbd8?, 0xc002596a20?}, {0x92ff0a2?, 0x4e4fc13?}, {0xc000b8dfc0?, 0xc00226f000?})
    net/http/transport.go:1183 +0xd2
net/http.(*Transport).dialConn(0xc002f9cb40, {0xae8bbd8, 0xc002596a20}, {{}, 0x0, {0x9301a85, 0x5}, {0xc000b8dfc0, 0x1a}, 0x0})
    net/http/transport.go:1625 +0x7e8
net/http.(*Transport).dialConnFor(0xae8bc10?, 0xc003340370)
    net/http/transport.go:1467 +0x9f
created by net/http.(*Transport).queueForDial in goroutine 6562349
    net/http/transport.go:1436 +0x3cb

```

* prevent modifying servers if changed

lib/web/app/transport.go

@@ -281,12 +281,18 @@ func (t *transport) DialContext(ctx context.Context, _, _ string) (conn net.Conn
 		break
 	}
 
-	// eliminate any servers from the head of the list that were unreachable
 	t.mu.Lock()
-	if i < len(servers) {
-		t.c.servers = t.c.servers[i:]
-	} else {
-		t.c.servers = nil
+	// Only attempt to tidy up the list of servers if they weren't altered
+	// while the dialing happened. Since the lock is only held initially when
+	// making the servers copy and released during the dials, another dial attempt
+	// may have already happened and modified the list of servers.
+	if len(servers) == len(t.c.servers) {
+		// eliminate any servers from the head of the list that were unreachable
+		if i < len(t.c.servers) {
+			t.c.servers = t.c.servers[i:]
+		} else {
+			t.c.servers = nil
+		}
 	}
 	t.mu.Unlock()