Segfault if client does not trust the proxy's SSL certificate

Greetings, and thanks for providing this library.

While getting set up with the examples, I managed to crash the proxy server because I did not pass the CA cert to curl.

```
$ curl -v -x https://user:pass@127.0.0.1:3333/ https://www.google.com/
* Trying 127.0.0.1:3333...
* Connected to 127.0.0.1 (127.0.0.1) port 3333 (#0)
* ALPN: offers http/1.1
* (304) (OUT), TLS handshake, Client hello (1):
* CAfile: /etc/ssl/cert.pem
* CApath: none
* (304) (IN), TLS handshake, Server hello (2):
* (304) (IN), TLS handshake, Unknown (8):
* (304) (IN), TLS handshake, Certificate (11):
* SSL certificate problem: self signed certificate in certificate chain
* Closing connection 0
curl: (60) SSL certificate problem: self signed certificate in certificate chain
More details here: https://curl.se/docs/sslcerts.html

curl failed to verify the legitimacy of the server and therefore could not
establish a secure connection to it. To learn more about this situation and
how to fix it, please visit the web page mentioned above.
```

Resulted in the following segfault:

```
$ go run examples/mitm/main.go
2023/10/20 11:56:57 55262#1 [debug] mitm: cache miss for 127.0.0.1
2023/10/20 11:56:57 55262#6 [info] start listening to [::]:3333
2023/10/20 11:56:59 55262#6 [debug] id=100001: accepted connection from 127.0.0.1:58335
2023/10/20 11:56:59 55262#9 [debug] id=100001: waiting for request
2023/10/20 11:56:59 55262#10 [debug] id=100001: failed to read request: remote error: tls: unknown certificate authority
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x40 pc=0x10468f04c]

goroutine 9 [running]:
github.com/AdguardTeam/gomitmproxy.(*Proxy).handleRequest(0x140001b2200, 0x1400006a940)
/Users/steve/go/github.com/gomitmproxy/proxy.go:232 +0x3c
github.com/AdguardTeam/gomitmproxy.(*Proxy).handleLoop(0x140001b2200, 0x0?)
/Users/steve/go/github.com/gomitmproxy/proxy.go:220 +0x70
github.com/AdguardTeam/gomitmproxy.(*Proxy).handleConnection(0x140001b2200, 0x1400006a940)
/Users/steve/go/github.com/gomitmproxy/proxy.go:208 +0x10c
created by github.com/AdguardTeam/gomitmproxy.(*Proxy).serve
/Users/steve/go/github.com/gomitmproxy/proxy.go:191 +0x34
exit status 2
```

The issue looks to be [this line of code](https://github.com/AdguardTeam/gomitmproxy/blob/147e33e57cbace9ea6b33a02b9479e010bcb288a/proxy.go#L232) which is dereferencing the request prior to checking `err`.

I'll submit a PR to fix.