Validate user input with roaring_bitmap_internal_validate

[andreas]

This PR adds roaring_bitmap_internal_validate to rb_from_bytea and roaringbitmap_in. From the documentation:

pg_roaringbitmap/roaring.h

Lines 2132 to 2145 in 75f84d4

	/**
	* Perform internal consistency checks. Returns true if the bitmap is
	* consistent. It may be useful to call this after deserializing bitmaps from
	* untrusted sources. If roaring_bitmap_internal_validate returns true, then the
	* bitmap should be consistent and can be trusted not to cause crashes or memory
	* corruption.
	*
	* Note that some operations intentionally leave bitmaps in an inconsistent
	* state temporarily, for example, `roaring_bitmap_lazy_*` functions, until
	* `roaring_bitmap_repair_after_lazy` is called.
	*
	* If reason is non-null, it will be set to a string describing the first
	* inconsistency found if any.
	*/

See #42.