Fix warning on windows: signed/unsigned mismatch on lines 415, 454

Comparison was between size_t data_sz >= int index_sz on 415 and
unsigned int data_sz >= int index_sz on 454. Both might be changed to
size_t but that would be tracing and replacing all comparisons is
outside the scope of this change.

In the rest of these two functions ensure unsigned values are used
consistently.

Change-Id: I922b399ceca612a92f44b9d1d331c1c6bae9d768