When String() was called on the maximum value of an integer type (eg
255 for uint8) this would cause an integer overflow, which would cause
an index error later in the code.
Fixed by re-arranging the code slightly.
Fixesgolang/go#10563
Change-Id: I9fd016afc5eea22adbc3843f6081091fd50deccf
Reviewed-on: https://go-review.googlesource.com/9255
Reviewed-by: Rob Pike <r@golang.org>
