Fix: Add assertion for n_ticks minimum when labels provided

[brenocq]

Fixes #600

Problem
When calling SetupAxisTicks(axis, v_min, v_max, n_ticks, labels) with n_ticks = 1 and a single-element labels array, the function:

1. Increases n_ticks to 2 (minimum requirement)
2. Accesses labels[1] which is out of bounds
3. Causes undefined behavior / potential crash in strlen()

Solution
Added assertion that validates when custom labels are provided, n_ticks must be at least 2.