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>