tree bccf3fc535fb3e5259b7294b4132c220e68451f0
parent 349cbfcf504901f4efad60a0f84b27d0bfb5d367
author Oliver Smith <osmith@sysmocom.de> 1612530920 +0100
committer Oliver Smith <osmith@sysmocom.de> 1612539422 +0100

apn_start: avoid segfault if missing tun-device

Check if tun-device is defined and give the user a hint that it is
missing instead of segfaulting with gtpu-mode kernel-gtp:
  20210205141701206 DGGSN <0002> ggsn.c:186 APN(internet): Starting
  20210205141701206 DGGSN <0002> ggsn.c:204 APN(internet): Opening Kernel GTP device (null)
  Segmentation fault

With gtpu-mode tun it didn't segfault, but still tried to open the NULL
device:
  20210205141557598 DGGSN <0002> ggsn.c:186 APN(internet): Starting
  20210205141557599 DGGSN <0002> ggsn.c:189 APN(internet): Opening TUN device (null)
  20210205141557599 DTUN <0001> tun.c:195 errno=1/Operation not permitted ioctl() failed

Related: OS#3208
Change-Id: I9f71af65cc0eed71728c04b774e5c08352947913