netsniff-ng: Fix compiler warning

Fix the following compiler warning that occurs when building with "-W
-Wall -Wextra" by introducing a cast:

netsniff-ng.c: In function ‘walk_t3_block’:
netsniff-ng.c:841:8: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>