Fix memory corruption issues in TextLayoutCache. (DO NOT MERGE)

Ensure log_clusters array is big enough.
Bug: 5714171

Explicitly handle the cases where the entire string or a single
run might have a length of 0.
Harfbuzz assumes the length of the item is at least 1.
If the length is zero, then it will clobber memory at index -1
into the log_clusters array.
Bug: 5705479

Change-Id: If28a9866221081f69973c1d12d7fe0cf8db2edd0