encoding/hex: add error check for write error.

I believe the original author of this code just forgot to check for error here.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/76760043

src/pkg/encoding/hex/hex.go

@@ -146,6 +146,9 @@ func (h *dumper) Write(data []byte) (n int, err error) {
 			h.buf[12] = ' '
 			h.buf[13] = ' '
 			_, err = h.w.Write(h.buf[4:])
+			if err != nil {
+				return
+			}
 		}
 		Encode(h.buf[:], data[i:i+1])
 		h.buf[2] = ' '