efi_loader: fix 'efidebug boot dump'

* Do not recreate a variable name that we already have as u16 string.
* Check the return value of malloc()
* EFI_NOT_FOUND cannot occur for a variable name returned by
  GetNextVariableName(). Remove a print statement.
* Don't copy a GUID for no reason.
* Don't use the run-time service table to call exported functions.
* Don't pass NULL to show_efi_boot_opt_data() (fixes Coverity CID 300338).

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>