fix assert in __deregister_frame_info_bases

The assertion in __deregister_frame_info_bases assumes that for every
frame something was inserted into the lookup data structure by
__register_frame_info_bases. Unfortunately, this does not necessarily
hold true as the btree_insert call in __register_frame_info_bases will
not insert anything for empty ranges. Therefore, we need to explicitly
account for such empty ranges in the assertion as `ob` will be a null
pointer for such ranges, hence causing the assertion to fail.

Signed-off-by: Sören Tempel <soeren@soeren-tempel.net>

libgcc/ChangeLog:
	* unwind-dw2-fde.c: Accept empty ranges when deregistering frames.