The initial value of free memory blocks in 0. When the query cache is enabled 
a new memory block gets allocated and is assigned number 1. The free memory
block is later split each time query cache memory is allocated for new blocks.
This means that the free memory block counter won't be reduced to zero when
the number of allocated blocks are zero, but rather one. To avoid confusion
this patch changes this behavior so that the free memory block counter is
reset to zero when the query cache is disabled.
Note that when the query cache is enabled and resized the free memory block
counter was still calculated correctly.