Before this fix, the command
SHOW ENGINE PERFORMANCE_SCHEMA STATUS
could report wrong amount of memory allocated,
when the amount of memory used exceeds 4GB.
The problem is that size computations are not done using size_t,
so that overflows do occur, truncating the results.
This fix compute memory sizes properly with size_t.
Tested manually.
No test script provided, as the script would need to allocate too much
memory for the test.
