SPDY: do not try to access an uninitialized tvb 

Coming when data length = 0

Bug: 10704
Change-Id: Ibd0730be661d966aa0429480dba97ed7d3e5c471
Reviewed-on: https://code.wireshark.org/review/5377
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
(cherry picked from commit e97d235efcd718f17220d6a21c9037581a9df9ae)
Reviewed-on: https://code.wireshark.org/review/5383