ppd-conflicts.c: Fix SEGV in 'cupsResolveConflicts()'

[Drawishe]

A fuzzer found a way to call cupsResolveConflicts() with NULL options structure and not-null num_options. This caused a segmentation fault because non-existent elements of the options structure are accessed in cupsResolveConflicts() function. To avoid this, we need to add a NULL values check for the elements of the structure.

Fixes #896