block: Fix error path segfault in bdrv_open()

Using an invalid option for a block device that is opened with
BDRV_O_PROTOCOL led to drv = NULL, and when trying to include the driver
name in the error message, qemu dereferenced it:

    $ x86_64-softmmu/qemu-system-x86_64 -drive file=/tmp/test.qcow2,file.foo=bar
    Segmentation fault (core dumped)

With this patch applied, the expected error message is printed:

    $ x86_64-softmmu/qemu-system-x86_64 -drive file=/tmp/test.qcow2,file.foo=bar
    qemu-system-x86_64: -drive file=/tmp/test.qcow2,file.foo=bar: could
    not open disk image /tmp/test.qcow2: Block protocol 'file' doesn't
    support the option 'foo'

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
