Fix overflow sanitizer in copyWithAuthor.

The array index calculation in NBLog.cpp causes a runtime error on
integer overflow sanitized builds. Although kPreviousLengthOffset is now
ssize_t, sizeof() returns size_t, which is causing kPreviousLengthOffset
to be implicitly cast to size_t, resulting in the overflow.

 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