erasure-code: use uintptr_t instead of long long

Checking the pointer alignment using a cast to long long raises a
warning when --Wpointer-to-int-cast is given.

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1076 from dachary/wip-vector-op

erasure-code: use uintptr_t instead of long long

Reviewed-by: Andreas Peters <andreas.joachim.peters@cern.ch>