Fix stdin issue for rust--format-fix-rustfmt-buffer.

[mookid]

fix #365
The issue seems to be that in case of whitespace issues, rustfmt emits the message

error[internal]: left behind trailing whitespace
  --> stdin:13:13:0
   |
13 |                 
   | ^^^^^^^^^^^^^^^^
   |

warning: rustfmt has failed to format. See previous 1 errors.

and the replacement code that looks for ":" fails to replace the reference. As a consequence, the code that jumps to the error pops a new buffer, which is wrong.

cc @xaon @Dushistov

[rust-highfive]

r? @brotzeit

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