Enforce null-termination in ResStringPool::stringAt

Rejects any non null-terminated string that a caller asks
ResStringPool::stringAt for, returning NULL instead.

The rationale for returning NULL rather than amending the string to add
a null-terminator is that conformant APK files will have all their
strings null-terminated anyway, and that this is a possible signal of a
malformed package.

Bug: 15288069
Change-Id: I370937b92f2cadf67fbd54203cbc7d1494be969f