BUG/MINOR: memprof: properly account for differences for realloc()

BUG/MINOR: memprof: properly account for differences for realloc()

The realloc() function checks if the size grew or reduced in order to
count an allocation or a free, but it does so with the absolute (new
or old) value instead of the difference, resulting in realloc() often
being credited for allocating too much.

No backport is needed.