cmd: efidebug: check pointer after memory allocation

After calling efi_alloc() we must check that the return value is not NULL.

Fixes: f01c961e ("cmd: efidebug: add uri device path")
Addresses-Coverity-ID: 477185 Dereference null return value
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>