Check for null-terminator in ResStringPool::string8At

All other stringAt methods check for null termination. Be consistent
so that upper levels don't end up with huge corrupt strings.

Bug: 62537081
Test: none
Change-Id: I17bdfb0c1e34507b66c6cad651bbdb12c5d4c417
(cherry picked from commit 3d35a0ea)
(cherry picked from commit 97f8cb01149b35b1832c7f9efe85ff19edf1083e)
(cherry picked from commit 5ec65ae9)