tvbparse: do not increment the token length when the new sub token length is 0

It fixes the unwanted addition of ignore characters when using tvbparse_some as seen with the capture from bug 10417

Change-Id: If68b587e1821fb34aa1fb56666986e1fa6385c33
Reviewed-on: https://code.wireshark.org/review/3905
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>