memtrace: Fix pointer arithmetic of MemoryShadow::cover

Pointers have unsigned type thus casting them to intptr_t
could lead to a negative value which in one part of the expression
is being implicitly converted to size_t resulting in:
 nBlocks = big_positive_number - negative_number

This rarely happens on x64 but would often happen on x32
resulting in memory access violation.

Fixes: bac0ea03

Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>