add err check

[dogancanbakir]

Closes #806

$ cat sess 
server-url: https://interact.sh

before:

$ go run .  -sf sess

    _       __                       __       __  
   (_)___  / /____  _________ ______/ /______/ /_ 
  / / __ \/ __/ _ \/ ___/ __ '/ ___/ __/ ___/ __ \
 / / / / / /_/  __/ /  / /_/ / /__/ /_(__  ) / / /
/_/_/ /_/\__/\___/_/   \__,_/\___/\__/____/_/ /_/

                projectdiscovery.io

[INF] Current interactsh version 1.1.9 (latest)
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x18 pc=0x10545ec7c]

goroutine 1 [running]:
github.com/projectdiscovery/interactsh/pkg/client.decodePublicKey({0x0?, 0x0?})
        /Users/fortytwo/Projects/interactsh/pkg/client/client.go:284 +0x3c
github.com/projectdiscovery/interactsh/pkg/client.New(0x140006d3ed0)
        /Users/fortytwo/Projects/interactsh/pkg/client/client.go:169 +0x44c
main.main()
        /Users/fortytwo/Projects/interactsh/cmd/interactsh-client/main.go:133 +0x13f0
exit status 2

after:

$ go run .  -sf sess

    _       __                       __       __  
   (_)___  / /____  _________ ______/ /______/ /_ 
  / / __ \/ __/ _ \/ ___/ __ '/ ___/ __/ ___/ __ \
 / / / / / /_/  __/ /  / /_/ / /__/ /_(__  ) / / /
/_/_/ /_/\__/\___/_/   \__,_/\___/\__/____/_/ /_/

                projectdiscovery.io

[INF] Current interactsh version 1.1.9 (latest)
[FTL] Could not create client: [:RUNTIME] failed to decode public key <- failed to decode PEM block containing the public key
exit status 1

P.S. This was the only way I could repro the issue.