Fixing NULL pointer access causing segmentation fault in aapt

In cases where resource packages are generated which does
not have public resources for all resource types - NULL
pointers sneaks into the mOrderedConfigs vector causing
the aapt tool to crash. The solution in this fix simply
ignores the NULL elements when later processing the sorted
vector.

The NULL elements are inserted during applyPublicEntryOrder
function call

Change-Id: I8b7135932e2d235018da2e2f7e09f0d22f1b2f59