efi_loader: add protection for block_dev

Check the value of block_dev before to use this pointer.

This patch solves problem for the command "load" when ubifs
is previously mounted: in this case the function
blk_get_device_part_str("ubi 0") don't return error but return
block_dev = NULL and then data abort.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>