The EncodeRune test exercises DecodeRune, but only for runes that it can encode. Add an explicit test for invalid utf16 surrogate pairs.

Bonus: coverage is now 100%

unicode/utf16/utf16.go: IsSurrogate     100.0%
unicode/utf16/utf16.go: DecodeRune      100.0%
unicode/utf16/utf16.go: EncodeRune      100.0%
unicode/utf16/utf16.go: Encode          100.0%
unicode/utf16/utf16.go: Decode          100.0%
total:                  (statements)    100.0%

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/39150044