cmd/serv panics on git ssh command to non-existent/unauthorized repo

[aswild]

• Gitea version (or commit ref): master (50d8d17)
• Git version: n/a
• Operating system: Linux
• Database: n/a
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL) See description
  • No
  • Not relevant
• Log gist:

Description

The serv command panics when trying to run a git SSH command on a non-existent or unauthorized repo. A go panic and stack trace is returned to the client, leaking server and build system information.

% git ls-remote ssh://git@try.gitea.io/aswild/test1 
31430109d60ef9dfe85d38b278c997d5db4285d2        HEAD
31430109d60ef9dfe85d38b278c997d5db4285d2        refs/heads/master

% git ls-remote ssh://git@try.gitea.io/aswild/invalid
Gitea: Unauthorized
Cannot find repository aswild/invalid
Gitea: Internal Server Error
Cannot find repository aswild/invalid
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x16531b9]

goroutine 1 [running]:
code.gitea.io/gitea/cmd.runServ(0xc0000cadc0, 0x0, 0x0)
        /go/src/code.gitea.io/gitea/cmd/serv.go:188 +0xa69
github.com/urfave/cli.HandleAction(0x18add20, 0x291ed90, 0xc0000cadc0, 0xc0007f0600, 0x0)
        /go/src/code.gitea.io/gitea/vendor/github.com/urfave/cli/app.go:490 +0xc8
github.com/urfave/cli.Command.Run(0x1b22ea2, 0x4, 0x0, 0x0, 0x0, 0x0, 0x0, 0x1b8bc2b, 0x2f, 0x0, ...)
        /go/src/code.gitea.io/gitea/vendor/github.com/urfave/cli/command.go:210 +0x996
github.com/urfave/cli.(*App).Run(0xc0007fe1a0, 0xc00003c0c0, 0x4, 0x4, 0x0, 0x0)
        /go/src/code.gitea.io/gitea/vendor/github.com/urfave/cli/app.go:255 +0x6af
main.main()
        /go/src/code.gitea.io/gitea/main.go:109 +0x811
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

This looks like a regression in 356854fc5. The log.GitLogger.Fatal call used run os.Exit(1) to exit the serv command, but when that was removed, the function continues normally after the fatal error, causing a panic.