vpx_mem,align_addr: use ~ to create mask

removes the need for an intermediate cast to int, which was missing in
the call added in:
69c5ba1 vpx_mem: Refactor code

quiets a visual studio warning:
C4146: unary minus operator applied to unsigned type, result still
unsigned

Change-Id: I76c4003416759c6c76b78f74de7c0d2ba5071216
2 files changed