efi_loader: capsule: Record capsule result only if capsule is read

Record capsule update result only if the capsule file is
successfully read, because the capsule GUID is not sure when
the file can not be read or the file is not a capsule.
Without this fix, if user puts a dummy (non-capsule) file
under (ESP)EFI/UpdateCapsule, U-Boot causes a synchronous
abort.

This also fixes use-after-free bug of the 'capsule' variable.

Fixes: c74cd8bd ("efi_loader: capsule: add capsule_on_disk support")
Signed-off-by: Masami Hiramatsu <masami.hiramatsu@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>