GTPv2: fix an infinite loop

Bug: 10548
Change-Id: I916e15cecf0ec9dc607362b902709567d69243df
Reviewed-on: https://code.wireshark.org/review/4578
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Pascal Quantin <pascal.quantin@gmail.com>