Fix for backslashes being replaced by null bytes

[RafBishopFox]

This PR addresses #46. While testing Sliver, we noticed that the \ character was being consumed but not printed. Upon further investigation, it looks like backslashes were being replaced by null bytes (\x00). I tracked it down to the unescapeRunes function in inputrc/parse.go.

When the sequence of runes has a length of 1, the loop that reads ahead grabs zeroes for char1 through char5 and essentially interprets the sequence as trying to escape a null byte.

This PR returns a string representation of the sequence immediately if the length of the sequence is 1 to avoid this.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a558922) 89.29% compared to head (44a863d) 89.30%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #61   +/-   ##
=======================================
  Coverage   89.29%   89.30%           
=======================================
  Files          10       10           
  Lines        4736     4739    +3     
=======================================
+ Hits         4229     4232    +3     
  Misses        470      470           
  Partials       37       37           

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