Fix crash when package_add_command is missing.

When using packages promise and not defining 'package_add_command'
agent crashes. This fixes crash.
(cherry picked from commit 163086eda00ef930553050aed2afa875f2bdb564)

Conflicts:

	cf-agent/verify_packages.c
Merge pull request #2060 from pasinskim/3.6.x

Fix crash when package_add_command is missing.