Fix overflow sanitizer in copyWithAuthor.

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

 buffer[sizeof(buffer) + Entry::kPreviousLengthOffset] = ...

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

This changes kPreviousLengthOffset to signed, which provides the same
effect without the overflow.

Bug: 30969751
Test: Compiles, device boots.
Change-Id: I25b232bba6a1940674d15a06483b2595eecc29e8