block: fix dangling bs->explicit_options in block.c

In some error paths it is possible to QDECREF a freed dangling
explicit_options, resulting in a heap overflow crash.  For example
bdrv_open_inherit()'s fail unrefs it, then calls bdrv_unref which calls
bdrv_close which also unrefs it.

Signed-off-by: Manos Pitsidianakis <el13635@mail.ntua.gr>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
