efi_loader: parameter check in GetNextVariableName()

If GetNextVariableName() is called with a non-existing combination of
VariableName and VendorGuid, return EFI_INVALID_PARAMETER.

If GetNextVariableName() is called with a string that is not zero
terminated, return EFI_INVALID_PARAMETER.

Reformat a line over 80 characters.

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