libext2fs: sanity check group argument to ext2fs_clear_{block,inode}_uninit()

libext2fs: sanity check group argument to ext2fs_clear_{block,inode}_uninit()

Avoid a potential out-of-bounds memory access if the group passed to
ext2fs_clear_block_uninit() or ext2fs_clear_inode_uninit() is greater
than the number of groups in the file system.  This prevents a failure
in resize2fs when to allocate a block when growing the file system
significantly.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>