Fixed truncation issue

[Aym3nTN]

#76
"
When trying to truncate a string with a value greater than its size, we get the below error.

Example of failed test: {"DOBROSLAWZYBORT", 17, "dobroslawzybort", false}

--- FAIL: TestSlugMakeSmartTruncate (0.01s)
panic: runtime error: slice bounds out of range [:17] with length 15 [recovered]
	panic: runtime error: slice bounds out of range [:17] with length 15

"

[codecov]

Codecov Report

Merging #77 (d046b02) into master (e79300e) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #77   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines          142       142           
=========================================
  Hits           142       142           

Impacted Files	Coverage Δ
slug.go	100.00% <100.00%> (ø)

[matrixik]

And here I though that I added enough new test cases.
Thank you for finding and fixing it.