strconv: add a test case when base is illegal
strconv: add a test case when base is illegal

Increase unit test coverage of strconv/itoa.go from 83.8% to 85%

Change-Id: I0abdfc70a3f06251106f229c12ffb105eeb43fc8
GitHub-Last-Rev: da9755368b5db41123c8f945fecedd135e9f894a
GitHub-Pull-Request: golang/go#55014
Reviewed-on: https://go-review.googlesource.com/c/go/+/430137
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Run-TryBot: Jenny Rakoczy <6a3f4389a53c889b623e67f385f28ab8e84e5029@golang.org>
Reviewed-by: Jenny Rakoczy <6a3f4389a53c889b623e67f385f28ab8e84e5029@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Auto-Submit: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Auto-Submit: Jenny Rakoczy <6a3f4389a53c889b623e67f385f28ab8e84e5029@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go