Handle missing line info in printcontext

before:

    $ ./dlv -run
    Type 'help' for list of commands.
    dlv> step
    Stopped at: :0
    Command failed: open : no such file or directory
    dlv> step
    Stopped at: :0
    Command failed: open : no such file or directory

after:

    $ ./dlv -run
    Type 'help' for list of commands.
    dlv> step
    Stopped at: 0x7fb7c452b334
    =>    no source available
    dlv> step
    Stopped at: 0x7fb7c452b337
    =>    no source available