* Make intermediate arrays in av1(_highbd)_warp_affine_c signed,
  to avoid integer overflow when multiplying an 'unsigned int'
  by a negative 'int' value.

* Pad out arrays in masked_variance_test.cc so that the array
  stride is a multiple of 16 bytes.
  This fixes some UBSan errors in masked_variance_intrin_ssse3.c
  related to unaligned loads of 32-bit values.

BUG=aomedia:572

Change-Id: I0cf786c94870ff128c883bed8e900b0686afc3f7