Fix test suite crashes with libobjc2 runtime.

libobjc2 has a notion of "small objects". The type
of a small object is detected by checking its address:
it should not be word-aligned.

When performing a custom memory allocation for Request,
make sure it's not a small object, by aligning it around
(void *) pointer (word) boundary.

object_dispose in libobjc2 runtime doesn't check whether
the passed in object is nil, so check for nil outside.