Check for invalid (negative) string pool pos.

Parcel implementation would crash the system server otherwise.
Any exception is OK as it'll cause the cache to be invalidated.
Crash is not OK.

Bug: 194632313
Fixes: 194632313
Test: atest PackageParserCacheHelperTest
Change-Id: I58a4496b4646e172e6c3aee9ea17854a7ef55eaa