Fix overflow sanitizer in copyWithAuthor.

The array index calculation in NBLog.cpp causes a runtime error on
integer overflow sanitized builds.

 runtime error: unsigned integer overflow: 27 + 4294967295 cannot be
 represented in type 'unsigned int'

This restructures the buffer index to avoid the overflow all together.

Bug: 30969751
Test: Compiles, device boots.
Change-Id: I9abb858190ecdeeaede66cf502ceb586467d5c0c
Merged-In: I9abb858190ecdeeaede66cf502ceb586467d5c0c
(cherry picked from commit 9ef855d0)