src: Check ebl_openbackend result before using ebl handle.

src: Check ebl_openbackend result before using ebl handle.

GCC10 -fanalyzer plus -flto sees that ebl_openbackend can fail and
return NULL. Most of the time we will get a dummy ebl, but in case
of out of memory or corrupt ELF header it might return NULL. Make
sure that we report a (fatal) error in that case in all tools.

Signed-off-by: Mark Wielaard <mark@klomp.org>