blocks/reedsolomon: fix s_nz multi-driver in rs_syndrome

[kroening]

Summary

rs_syndrome.s_nz is assigned in two separate always @(posedge clk)
blocks: the main accumulator block resets it to 0 alongside
s_valid/acc, and a second, dedicated block both resets it to 0 and
drives its real value (the OR-reduce of the final syndromes) on
in_valid && in_last. A variable driven by more than one procedural
block is invalid Verilog, caught by any tool that checks for this --
e.g. ebmc reports `s_nz' has multiple drivers and rejects the
module.

Fix

Remove the redundant reset assignment to s_nz from the first block;
the second block already resets it on rst and is its sole driver.
No behavioral change (both blocks reset it to the same value 0; only
the second block ever drives it to anything else).

Note

reedsolomon still won't fully elaborate with ebmc after this fix --
rs_dec.v shares an integer loop counter (k) across two clocked
always blocks, which is a separate, real bug in ebmc itself (not
this RTL), already reported at
diffblue/hw-cbmc#2184.

How found

Found via ebmc's parse/elaborate pass over the benchmark suite,
reported at https://diffblue.github.io/hw-cbmc/logikbench/.