bufio: remove unnecessary "continue"

After resizing the scan buffer, we can immediately read into the
newly-resized buffer since we know there is now space.

Fixes #15712.

Change-Id: I56fcfaeb67045ee753a012c37883aa7c81b6e877
Reviewed-on: https://go-review.googlesource.com/31715
Run-TryBot: Quentin Smith <quentin@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/bufio/scan.go

@@ -199,7 +199,6 @@ func (s *Scanner) Scan() bool {
 			s.buf = newBuf
 			s.end -= s.start
 			s.start = 0
-			continue
 		}
 		// Finally we can read some input. Make sure we don't get stuck with
 		// a misbehaving Reader. Officially we don't need to do this, but let's