Don't panic in `threads` if there is no info for pc

`threads` currently panics in proctl.ThreadContext.PrintInfo when PCToLine
can't find any info for the current pc.

    $ ./dlv -run
    Type 'help' for list of commands.
    dlv> threads
    panic: runtime error: invalid memory address or nil pointer dereference
    [signal 0xb code=0x1 addr=0x8 pc=0x47f00b]
    
    goroutine 1 [running, locked to thread]:
    github.com/derekparker/delve/proctl.(*ThreadContext).PrintInfo(0xc20800a5a0, 0x0, 0x0)
            src/github.com/derekparker/delve/proctl/threads_linux_amd64.go:59 +0x29b
    [...]