net: pkt: Fix possible division by zero

When net debugging is enabled, the count variable is initialized to -1.
This may cause division by zero if there is only one fragment in pkt.
Solve this by setting the count to 0 and checking the value before the
print at the end of the function.

Successfully tested on STM32F407 SoC.

Signed-off-by: Ding Tao <miyatsu@qq.com>