fix rsession crash in debugger startup when source ref SEXP is null
Merge pull request #61 from rstudio/bugs/debugging

Fix rsession crash in debugger startup when source ref SEXP is null
avoid use of paste0 (not present in older versions of R)
Merge pull request #62 from rstudio/bugs/debugging

Avoid use of paste0 (not present in older versions of R)
Merge remote-tracking branch 'origin/master' into v0.98-shiny