BR 3392906: error out on bad syntax "db 1 2"

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>

include/nasm.h

@@ -316,6 +316,8 @@ struct tokenval {
     int64_t             t_inttwo;
     enum token_type     t_type;
     int8_t              t_flag;
+    const char		*t_start; /* Pointer to token in input buffer */
+    int			t_len;    /* Length of token in input buffer */
 };
 typedef int (*scanner)(void *private_data, struct tokenval *tv);