Fix crash in CosetLeadersMatFFE

Reject matrices with dependent rows before building the
internal search tables for CosetLeadersMatFFE. This avoids
a crash in the 8-bit path and restores correct behaviour
for square full-rank inputs.

AI assistance: Codex reproduced the crash, traced it to
the internal row-independence assumption, added a
regression test, and prepared the fix.

Co-authored-by: Codex <codex@openai.com>

Author: fingolfin

lib/listcoef.gi

@@ -1516,10 +1516,14 @@ InstallMethod(CosetLeadersMatFFE,"generic",IsCollsElms,
     q := Size(f);
     n := Length(mat[1]);
     m := Length(mat);
+    # The search below assumes the rows form a basis for an m-dimensional
+    # syndrome space, so reject dependent input before building the column data.
+    if RankMat(mat) <> m then
+        Error("CosetLeadersMatFFE: <mat> must have linearly independent rows");
+    fi;
     tofind := q^m;
     t := TransposedMat(mat);
     vl := [];
-    vl[m+1] := false;
     felts := AsSSortedList(f);
     Assert(2, felts[1] = Zero(f));
     nzfelts := felts{[2..q]};

tst/testbugfix/2026-04-15-CosetLeadersMatFFE-errors.tst

@@ -0,0 +1,8 @@
+# Regression test for square matrices and invalid dependent input in
+# CosetLeadersMatFFE.
+gap> M := IdentityMat(2, GF(2));;
+gap> L := CosetLeadersMatFFE(M, GF(2));;
+gap> Length(L);
+4
+gap> CosetLeadersMatFFE(L, GF(2));
+Error, CosetLeadersMatFFE: <mat> must have linearly independent rows