cat: panic when running with -T

[karlmcdowall]

The following fails...

$ /usr/bin/echo -en "\ta" | ./target/release/cat -T
thread 'main' panicked at src/uu/cat/src/cat.rs:546:17:
assertion `left == right` failed
  left: 97
 right: 10
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Basically, if there is a \t in a buffer that does NOT contain a subsequent \n and you're running with the -T flag it'll crash.

Fix is up for review here