Fix the "does break strict-aliasing rules" warning

This warning breaks the standalone compilation because of -Werror.
CA-98950: fix coalescing with small ancestors.

When the virtual size of the coalesce target's ancestors (grandparent & above,
since the parent's size must match the target's) is smaller (due to a resize),
vhd-util coalesce might incorrectly error out with ERANGE when reading a
sector(s) in a partial block beyond the smaller ancestor's size.
Merge pull request #23 from andreil/ca98950

CA-98950: fix coalescing in the presence of smaller ancestors