Panic when calling AccuracyMutatingLetters()

[douglasm]

When testing some data and calling cm.AccuracyMutatingLetters() I am getting a panic of:
panic: invalid argument to Intn

math/rand.(*Rand).Intn(0xc000080180, 0x0, 0x2)
/usr/local/go/src/math/rand/rand.go:169 +0x9c
math/rand.Intn(0x0, 0x2)
/usr/local/go/src/math/rand/rand.go:329 +0x37
github.com/schollz/closestmatch.(*ClosestMatch).AccuracyMutatingLetters(0xc0005354d0, 0x10)
/Users/imac/go/src/github.com/schollz/closestmatch/closestmatch.go:309 +0x2b2

My call is:

	cm := closestmatch.New(possible, subsetSize)
	result := cm.Closest("Rose Villa")

where:

	subsetSize := []int{2, 3, 4}
	possible := []string{"Chris Horsburgh's B&B", "R & R Boutique and Breakfast", 
                          "Oaklands Guest House", "Guthrie Bed and Breakfast",
		          "Arndean Bed & Breakfast", "Duntrune House B & B",
                           "Cortachy House Bed & Breakfast", "Brucefield Boutique Bed and Breakfast",
		          "Rose Villa B&B", "Crepto Bed and Breakfast", "Newton Farm Holidays",
                           "West Adamston Farmhouse", "Liscara Bed & Breakfast",
		          "Chapel House B & B", "Kinnaber B & B", "Maulesbank Guest House",
                           "Fallady Bank Bed and Breakfast", "Church House Bed & Breakfast",
		          "Bridgelee Bed and Breakfast", "Croftsmuir B & B", "Denbrae Cottage B&B",
                           "Bubbly Oak (The) ", "Purgavie Farmhouse Bed and Breakfast ",
		          "Muirhouses Farm Bed and Breakfast", "Five Gables House B&B"}

On examining the code for AccuracyMutatingLetters(), at line 283 of closestmatch.go i is being incremented before being compared to the random integer returned from rand.IntN. When the returned random number is 0 the i != testStringNum will never pass and as such the testString is always blank. When you get to line 309 where a rand.IntN is returned using the length of the testString as the parameter you get the panic as rand.IntN does not accept 0 as a valid value.

I got the code to run by moving the i++ line to after the i != testStringNum compare, but I do not know if this fix will cause the function to return inaccurate results.

There is also a i++ before a comparison in the AccuracyMutatingWords function at line 208.

Hope this all helps, let me know if you need more information.