Commits on Jan 23, 2018

1. Avoid underflow in render_source_line …

   While testing #47655 I was able to make the compiler panic
   when it's compiled with debug assertions:
   
   ```shell
   > rustc /dev/null --crate-type proc-macro
   
   error: internal compiler error: unexpected panic
   
   note: the compiler unexpectedly panicked. this is a bug.
   
   note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
   
   note: rustc 1.25.0-dev running on x86_64-apple-darwin
   
   note: run with `RUST_BACKTRACE=1` for a backtrace
   
   thread 'rustc' panicked at 'attempt to subtract with overflow', librustc_errors/emitter.rs:287:49
   ```
   
   Without debug assertions the following warning is emitted:
   
   ```shell
   > rustc /dev/null --crate-type proc-macro
   
   warning: unused variable: `registrar`
    --> /dev/null:0:1
     |
     |
     = note: #[warn(unused_variables)] on by default
     = note: to avoid this warning, consider using `_registrar` instead
   ```
   
   The panic is due to the unused variable warning being spanned to
   `/dev/null:0:1`. When `render_source_line` subtracts 1 from the line
   number to look up the source line it panics due to underflow. Without
   debug assertions this would wrap and cause us to return a blank string
   instead.
   
   Fix by explicitly testing for 0 and exiting early. I'm unsure how to
   automatically test this now that #46655 has been approved.

   

   etaoins committed Jan 23, 2018
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