Care for root index being undefine while calling Z3_algebraic_get_i()

In some cases, Z3_algebraic_get_i() returned 0. For example, in the following
Python snippet, the last assert would fail:

    import z3
    x = z3.Real('x')
    s = z3.Solver()
    s.add( (x * x) - 2 == 0, x <= 0)
    s.check()
    val_x = s.model().get_interp(x)
    assert val_x.index() == 1

The problem was that `algebraic_numbers::manager::imp::get_i()` did not
check whether the root index was properly initialized.

This commit fixes this issue by checking whether root index is initialized
the same way various other routines do.

Fixes issue Z3Prover#5807.

Signed-off-by: Jan Vrany <jan.vrany@labware.com>

src/math/polynomial/algebraic_numbers.cpp

@@ -2013,6 +2013,11 @@ namespace algebraic_numbers {
             }
             else {
                 algebraic_cell * c = a.to_algebraic();
+                if (c->m_i == 0) {
+                    // undefined
+                    c->m_i = upm().get_root_id(c->m_p_sz, c->m_p, lower(c)) + 1;
+                }
+                SASSERT(c->m_i > 0);
                 return c->m_i;
             }
         }