caputils: fix crash when calling 'dumpcap -i' without having WinPcap installed
Add a test on err_str buffer presence before trying to use it,
like what is done in other code paths.
Bug: 12143
Change-Id: I30ae49a33224dc190c202637767df9d7de2c0f2b
Reviewed-on: https://code.wireshark.org/review/14074
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>