assertion failed: bpos.to_u32() >= mbc.pos.to_u32() + mbc.bytes as u32

[dwrensha]

I'm seeing an internal compiler error on the following input (found by fuzz-rustc):

$ echo ZW51bQBlbQDLgsuC | base64 --decode > main.rs
$ rustc main.rs
error: unknown start of token: \u{0}
 --> main.rs:1:5
  |
1 | enumem˂˂
  |     ^

error: unknown start of token: \u{0}
 --> main.rs:1:8
  |
1 | enumem˂˂
  |       ^

error: unknown start of token: \u{2c2}
 --> main.rs:1:9
  |
1 | enumem˂˂
  |       ^
  |
help: Unicode character '˂' (Modifier Letter Left Arrowhead) looks like '<' (Less-Than Sign), but it is not
  |
1 | enumem<˂
  |       ^

error: unknown start of token: \u{2c2}
 --> main.rs:1:10
  |
1 | enumem˂˂
  |        ^
  |
help: Unicode character '˂' (Modifier Letter Left Arrowhead) looks like '<' (Less-Than Sign), but it is not
  |
1 | enumem˂<
  |        ^

thread 'rustc' panicked at 'assertion failed: bpos.to_u32() >= mbc.pos.to_u32() + mbc.bytes as u32', src/librustc_span/source_map.rs:840:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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 1.42.0-nightly (cd1ef390e 2020-01-31) running on x86_64-unknown-linux-gnu

error: aborting due to 4 previous errors

The same error happens on stable, beta, and nightly.