Fix range check to avoid index out of range panic

[divergentdave]

My server crashed with panic: runtime error: index out of range due to an incorrect range check. This PR should fix it by making sure there's enough buffer before reading a length value out of the SNI extension.

Error message and stack trace:

Mar 12 21:12:57 raspberrypi sniff[454]: 2017/03/12 21:12:57 Error: EOF
Mar 12 21:12:58 raspberrypi sniff[454]: 2017/03/12 21:12:58 Error: read tcp 192.175.111.254:5831: connection reset by peer
Mar 12 21:13:00 raspberrypi sniff[454]: 2017/03/12 21:13:00 Error: EOF
Mar 12 21:13:01 raspberrypi sniff[454]: 2017/03/12 21:13:01 Error: read tcp 192.175.111.254:26998: connection reset by peer
Mar 12 21:13:06 raspberrypi sniff[454]: panic: runtime error: index out of range
Mar 12 21:13:06 raspberrypi sniff[454]: goroutine 25485 [running]:
Mar 12 21:13:06 raspberrypi sniff[454]: pault.ag/go/sniff/parser.GetSNBlock(0x1046d0dc, 0x29, 0xf24, 0x0, 0x0, 0x0, 0x0, 0x0)
Mar 12 21:13:06 raspberrypi sniff[454]: /home/pi/go/src/pault.ag/go/sniff/parser/parser.go:91 +0x224
Mar 12 21:13:06 raspberrypi sniff[454]: pault.ag/go/sniff/parser.GetHostname(0x1046d000, 0x1000, 0x1000, 0x0, 0x0, 0x0, 0x0)
Mar 12 21:13:06 raspberrypi sniff[454]: /home/pi/go/src/pault.ag/go/sniff/parser/parser.go:42 +0xe0
Mar 12 21:13:06 raspberrypi sniff[454]: main.(*Proxy).Handle(0x104362a0, 0xb6d39a10, 0x1040a330)
Mar 12 21:13:06 raspberrypi sniff[454]: /home/pi/go/src/github.com/divergentdave/sniff/server.go:110 +0x174
Mar 12 21:13:06 raspberrypi sniff[454]: created by main.(*Config).Serve
Mar 12 21:13:06 raspberrypi sniff[454]: /home/pi/go/src/github.com/divergentdave/sniff/server.go:97 +0x2a0
...

[paultag]

This has some conflicts, mind resolving? Sorry for not seeing this, thanks for the PR!

[divergentdave]

Here you go!

[paultag]

Thank you! Longest time to merge ever.