libgomp: Fix reverse offload issues

If there is nothing to map, skip the mapping and avoid attempting to
copy 0 bytes from addrs, sizes and kinds.

Additionally, it could happen that a non-allocated address was deallocated,
such as a pointer set, leading to a free for the actual data.

libgomp/
	* target.c (gomp_target_rev): Handle mapnum == 0 and avoid
	freeing not allocated memory.
	* testsuite/libgomp.fortran/reverse-offload-6.f90: New test.