efi/libstub: Check return value of efi_parse_options

efi_parse_options can fail if it is unable to allocate space for a copy
of the command line. Check the return value to make sure it succeeded.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200430182843.2510180-12-nivedita@alum.mit.edu


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>