efi_loader: fix GetNextVariableName

Our current implementation of GetNextVariableName() first collects all EFI
variables. If none is found at all hexport_r() returns a zero length string
terminated by \0 and the value 1 as number of bytes in the returned buffer.

In this case GetNextVariableName() has to return EFI_NOT_FOUND.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>