arm/filter_neon_intrinsics.c: if the for loop end conditions
A classic programming error, the fix certainly works when fp is incremented by
16 and I believe it works for the += 12 cases because in those cases the data is
RGB, so the loop will not read beyond end of row.  (In the 16 byte case it does
read beyond end-of-row, but that's ok because the row buffers are guaranteed
padded.)

Signed-off-by: John Bowler <jbowler@acm.org>