os: remove a redundant branch in File.Read.

All implementations of File.read ensure that n >= 0. This is usually via
fixCount, except for Windows console reads, which only ever add to n.

Change-Id: Ic019d6a2da5ef1ac68d2690c908deca4fcc6b4a4
Reviewed-on: https://go-review.googlesource.com/12624Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/file.go

@@ -93,9 +93,6 @@ func (f *File) Read(b []byte) (n int, err error) {
 		return 0, ErrInvalid
 	}
 	n, e := f.read(b)
-	if n < 0 {
-		n = 0
-	}
 	if n == 0 && len(b) > 0 && e == nil {
 		return 0, io.EOF
 	}