The corruption ratio is calculated as the number of corrupted pages
divided by the number of pages that are not all-zeros. If all the
pages are all-zeros, it is calculated as 0/0, which usually returns
NaN or -NaN depending on compiler and optimization levels.

Clang's UBSAN warns that division by zero is undefined behavior.
Return NaN directly if all the pages are all-zeros to avoid the
undefined behavior.

Change-Id: Idd64aea7d94efdef9cc7aa227eaee7d719bde81e