NASM would try to "eat the comma token" in db expressions, even for
cases where the token was not a comma. Fix that and error out
properly.
To give better error messages, track where in the input string a token
starts or ends. This information is only valid as long as the input
string is kept, but that is just fine for error messages during
parsing.
Reported-by: Peter Cordes <pcordes@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
