rslib: decode_rs: Fix length parameter check

The length of the data load must be at least one. Or in other words,
there must be room for at least 1 data and nroots parity symbols after
shortening the RS code.

Signed-off-by: Ferdinand Blomqvist <ferdinand.blomqvist@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190620141039.9874-4-ferdinand.blomqvist@gmail.com