cmp_problem_data: use 0 for value if element is missing

NULL element value causes crashes but NULL is used only if element is
missing. Therefore we can make a compromise that NULL means 0.

Related to abrt/abrt#650

Signed-off-by: Jakub Filak <jfilak@redhat.com>
Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>