When a SyntaxError occurs, report the current offset within the stream.
The code already accounted for the offset within the current buffer
being scanned. By including how much data was already scanned, the
current offset can be computed.

Fixes #22478

Change-Id: I91ecd4cad0b85a5c1556bc597f3ee914e769af01
Reviewed-on: https://go-review.googlesource.com/74251Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>