[MaL] Petsc; Assert non-negative indices in DirBC

Negative indices mean ghost nodes for which the nodes
owning partition is responsible.
Might be helpful when implementing new BCs.

MathLib/LinAlg/PETSc/PETScTools.cpp

@@ -24,6 +24,16 @@ void applyKnownSolution(PETScMatrix& A, PETScVector& b, PETScVector& x,
 {
     A.finalizeAssembly();
 
+#ifndef NDEBUG
+    if (std::any_of(vec_knownX_id.begin(), vec_knownX_id.end(),
+                    [](PetscInt const i) { return i < 0; }))
+    {
+        OGS_FATAL(
+            "Found negative indices in the vector of Dirichlet boundary "
+            "conditions.");
+    }
+#endif  // NDEBUG
+
     A.setRowsColumnsZero(vec_knownX_id);
     A.finalizeAssembly();