leak-detective: Make sure to actually call malloc() from calloc() hook

leak-detective: Make sure to actually call malloc() from calloc() hook

Newer versions of GCC are too "smart" and replace a call to malloc(X)
followed by a call to memset(0,X) with a call co calloc(), which obviously
results in an infinite loop when it does that in our own calloc()
implementation.  Using `volatile` for the variable storing the total size
prevents the optimization and we actually call malloc().